Elevatoring - Using an item (usually a flare or food item in T2, or stacks of keys in TG) to fly/climb anywhere in the map.
Detailed explanation: https://www.ttlg.com/forums/showthread.php?t=58501
Keyholing - Collecting items or accessing switches through locked doors. Player needs to position himself near the door in such a way that items or switches he means are behind where keyhold would be or behind the line where door and wall meet. With some leaning forward and moving mouse around player will notice that door will lose the highlight even though its in front of the player. That means the object behind is highlighted. This is how you access it. I can provide a video if requested.
Door-leaning - I'm not sure whay this works and how to do it with great consistency. But if I run at the certain doors then stop a bit before them and immediatelly lean forward I can sometimes clip through the door, but only with head/camera. However this allows for accessing switches or loot behind the doors even if its positioned at a grater distance from the door.
Dolphining - Not exactly an exploit but still a usefull trick. Sometimes edge of the terrain is too far up from water for Garrett to be able to mantle up. If you swim down into the water then swim back up towards the edge, you can achieve a higher jump out of the water and reach edges otherwise unreachable.
Floater-jumping - Not strictly an exploit but not too far from it. Sometimes edge of the terrain is too far up from water for Garrett to be able to mantle up. If you drop an item that floats in water, like a scroll or a creat near the the edge, you can mantle up the item and then immediatelly jump forward. This will allow you to reach edges otherwise unreachable.
Door-glitching - Same technique as with Gentle glitching but done with blackjack in block position. Works best in T2. Experts like Psychosis would be able to provide more detail on this matter.
Rope-arrow-leaps - If one jumps on the very top of an achored rope arrow in a very precise way, Garrett will be flung a great distance in the direction of arrow end.
Slowfall-jumping - Another non glitch, yet awsome trick. One can use slowfall potion to jump higher.