The kind of stuff I remember doing:
1. Login with a temporary id, challenge some pro guy that he can't take me out in proper style. Loose to him but make notes of all his tricks and screw his happiness the next time with my main id.
2. Text walkthroughs. forgot the number of times I used alt+tab
3. Online weapon info. I used to hunt sites for every FPS game on how the weapon works in the game, like recoil rate, range, issues, bullet power, reload time, etc and then look up level data like map size, length of certain places, environmental factors in the map, etc and then pick up the right gun... Never spent this much time on my school books
4. Camping. Before starting with online gaming, I realized that bots were too dumb to fish out campers and figure out camping patterns. Often used the same place over and over again to dominate entire maps. In Unreal Tournament where I used to camp next to the sniper rifle in one level.
5. Stealth. This started with online FPS. Used max volume and headphones to identify sounds made by other people and abused silencors, snipers and meelee weapons to the max. Spent most of my time prowling in the background while people mostly used an on-the-face attack pattern.
6. Hide and fire. Used this extensively in games with poor AI or tough bosses. Especially Project IGI. The bots only notice you when you are closer than a point. So I used to take them out from a distance.
7. Binocular Zoom and fire. Completely throwing away real-life logic, used binoculars to zoom in on stationary (or apparently stationary) enemies, leave hand from mouse, switch to assault rifle and fire. This way saved up ammo and avoided using sniper much.
8. Trainers. GTA Vice City. I used this trainer mod vc_lights that caused lots of lights to come up in the sky over places where there were hidden packages. Only used this AFTER my 60th package though, and even with this trainer it was bloody hard to locate hidden packages.
9. Screwed up hardware. I had an issue with a mouse that used to double or triple click instead of single click. This forum (during its buggy days) is full of my double posts from 2 years back due to that reason... A friend borrowed it to own at an FPS with a gun that did not have an automatic mode. He ended up using this newly found "burst mode" on my mouse a lot...