#1. Bunny hop. After every jump game stops player, that's prevent moving by frequent jumps. But if we jumps off exactly at the moment of landing then we avoids it. Easiest way to do it - to send big amount of jump commands for little time. We have two variants without using special programs or devices. First - wheel of mouse. By using commands like 'bind mwheeldown +jump' or 'bind mwheelup +jump' we sets jump to moving of wheel down or up. While turning of wheel big amount of jump commands occurs and we jumps off without speed decrease. Second variant is typing script like this: 'bind mouse2 ";+jump;wait;-jump;wait;+jump;wait;-jump;wait;+jump;wait;-jump;wait;+jump;wait;-jump;wait;+jump;wait;-jump;wait;+jump;wait;-jump;wait;+jump;wait;-jump"' that means many jump commands on right mouse button. You can do it for any other key. Small mistakes of landing you can offset by normal strafes. When you are in air, release 'forward', press side move key and synchronize mouse move to same side. After that similar to other side. If you'll do it well, you'll become not only not slower but much faster than usual running. 
#2. Elevator on beetle. Based on #1. Using script, you can make jumps on beetle and gain the height if close to tall wall. This bug working because beetles get some stuck under player's model, if they will appear under it. But for injuring and jumping off from player's model, beetle has to approach more close to player. Game has ability to jump off from beetles, so if you will continuously jump on beetle you will continuously gain the height until the wall or map will finish. 
#3. Gauss - boost. While shooting by alter fire you get huge impulse as a recoil. you can use it as strong horizontal speed boost while player locates in air. 
#4. Gauss - ricochet. The one gun which has ability to ricochet. 
#5. Gauss - wallbang. Similar to #4.
#6. Sent elevator to ceiling. When the player's model locates out the elevator, it prevents moving of elevator. If you'll actuate elevator and prevent its moving, then elevator will go up, despite of ceiling there.
#7. Ladder boost. Game engine defines move speed to each move-key separately. If you press two keys (not beyond each other) then while directing view to middle of two directions which relevant each move key you will get overlap of velocities ladder moving.
#8. Edge bug. At the time of landing player can loose speed by edge of any horizontal texture. If player immediately will continue the flight then game may not react to landing and not decrease health.
#9. Jump bug. Sitting model of player has smaller height. after releasing "duck" model immediately lines up. If at this moment you jump from ground then game can miss the moment of landing and health will not decreased. this point in time is so small that probability of executing bug is extremely low. Also it decreases by height.
#10. Hornetgun to gates. Based on #6. Any moving object can prevent other one as in bug 6. In this case, flying hornets are same objects. Also they infiltrate to gates enough deeply. Game engine forces these gates to move back if it meets obstacle. New map loads after complete closing gates so you can accelerate this process by hornets. 
#11. Rocket jump. Any explosion, close to player, gives him a huge impulse. You can use it as increasing both horizontal and vertical speed. 
#12. Cling to ladder. Player, clinging to ladder, immediately loose all speed without health lose. 
#13. Using through wall. Developers forgot to remove ability to actuate buttons and other devices through walls. 
#14. Beetle in teleport. It's working this way. Since the beginning teleport always works in this point. But there is a one limit that game can't teleport dead player. There is one trigger above the portal which give deadly impact to anybody who come in. It disappears after actuating portal in scenario. But if we'll throw the beetle, trigger will spend his strength to beetle and will not keep up to flying player followed. Eventually teleport will work.
#15. Lasermines to gates. These mines clings to any flat surface even it's moving. If you set mine that it will prevent to close gates so it will disarm and become ammo unit. In this case it appears inside the texture of gates and forces gates to get stuck until taking it.
#16. Break gates by yourself. Almost all objects in game which can be blown in story, for some reason can be broken by any other gun.
#17. Elevator ruse. Elevator doesn't moving anywhere in fact. When we actuates elevator, game checks our location. If we locates in trigger zone (in elevator) then after closing doors game switches the map while elevator seems like moving. But this trigger stick out the elevator for little. We can touch its edge and use it while we locates outside. 
#18. Beetle can eat the loot faster. Based on #2. Getting stuck ability of beetle can be useful for easy killing. Actually, beetle after bite takes impulse which throw it to side, because bites might not be too frequently. But if it locates already inside the model, then impulse does not work. So beetle quickly eat the loot. 
#19. Healing door. When player prevent moving any object, it can hurt or kill player. Speed of injuring depends on degree of preventing and parameters attributed by developers to each object. But by mistake they attributed increasing health (or armor if any). Game can display only 255 percent of health but there is no upper limit so you can gain health for much time. 
##20-26. (secrets)
#27. Breaking tanks by crowbar. Similar to #16.
##28-33. (secrets)
#34. How to come down without elevator. Player can climb on steep slopes by pressing move key to direction of this slope. Speed of climbing depends on steep degree. When player appears on this slope, speed becomes to this value for 1-2 seconds. So, if we get this slope while quick falling, we can press move key right to this slope and we'll quickly loose speed, even if we can't climb on top. 
#35. Vortigaunt didn't come back. Appearing vortigaunts falls into triggers that teleport them back. But if we move the trolley back, then one vortigaunt will fall on the trolley and will not reach his trigger. Eventually he will stay right there.
#36. Grenade into gap. The grenades like any other models has no physical size and has shape of point, but displays normally. Consequently, we can throw grenade into any thin gap. 
#37. Wagon got stuck. Grid has scenario to open and close in certain moments after wagon approaching. If wagon get stuck after approaching then it can close before it. It's not an obstacle for wagon but player can not go through it. You can force wagon to stick by snuggling to edges and corners while it turns. 
#38. Box flight. Game engine defines vertical speed to box if it meets a slope while flying. So it's normal. But developers forgot to add gravity effect after this flight. 
#39. Corpse can kill. Enough strange bug. Player can die exactly in this place if scientist is dead. 
#40. Jump-flat effects not only to player. This flat is a place with negative gravity parameters. So, it affects not only to player but any other object.
#41. Out the textures near rocket. There is hole left
#42. Enemy's missile managing. These missiles have program to direct by laser. And no matter whose laser it is.
#43. How to kill Nihilant easier. These guns have little other program in case of hitting enemy. It can impact the enemy's model and go inside the model. Here it can reach main crystal and kill Nihilant from floor at once. 
#44. Shortcut by rock near dam. If while player's flying two walls located by acute angle block him then player stops to fall. This can be used for saving health while falling from big height.
#45. Shortcut by button through wall. Based on #13. Using bug 13 we can actuate rocket from outside and get ready near airlock. It gives time economy.
#46. Shortcut - jump to fence. Based on #1. Using strafe we can reach the fence and shortcut.
#47. Shortcut near bunch of lasermines. Game allows to jump on armed mines and climb high impediments by that. 
#48. Shortcut on big rock Using ledge of texture we can go into place where we are unexpected.
#49. Shortcut - use through door. Based on #13.
#50. Using scientists and guards. Secret message of scientist. Game opens door facing scientist or guard if it has no eye-check lock. The one difficulty is to force him to approach door and make movement right into the door. You should do this by kicks to certain directions. Phrase of scientist should be after opening doors to blast pit. But developers maybe by mistake kills this scientist according story before that moment. 
#51. Object boost. At the time of pressing key 'use' on any movable object, game stops player and does not allow player to gain speed. But for short-term moment of time while we press 'use' in moving state, object takes huge acceleration and for a little time clings to player. If you'll get the moment when object's speed is big, but object has link with player yet and you'll release 'use' key, then player will quickly take off to same direction with object. 
#52. Trolley's skip. Based on #1, #51. Using maximum effectively Object boost with trolley, you should choose position in order for touch leaned surface while flying. It will force player to get huge height. There is one hidden trigger on the ceiling of test chamber. It changes map to next, which seems like previous but after incident. While flying you should use strafes and direct yourself to this trigger and further to edge of aggregate in top of chamber. You should do last part by memory because screen will fade to black. After completing, jump to window. Here is small invisible ledge which doesn't allow you to fall further down. Finally go into window and that's all.