There was some early hacks that read the wow memory and displayed it graphically next to the game. e.g. you could have a map showing you the locations of everything near you (that the game knew about but did not tell you). – required playing in windowed mode.
You had bots and advanced bots that would either click a specific point on the screen, or perform a slow operation where you could run between specific points on the map and fight anything on the way. Or more advanced ones using external application and internal UI functions to monitor your health etc values to have a more advanced botting experience.
Or teleport hacks that would move your z-position below or way above ground and warp you accross the continent, or to specific waypoints where chests and rare resources spawned (favoured by goldfarmers for a while).
You could also teleport below ground and attack people above ground, but they could not fight back.
Or Direct3D Hacks that read the game memory and gave you additional graphical functions in game, such as a radar showing you everything even if you were not a hunter so you could see enemies before they saw you, etc etc.
There were also a few dupes where you could enter and exit some instances and also perform some functions to dupe money (dont know enough about this unfortunately).
You also had the ability to walk along ‘seams’ on the mountain (mountainwalk) – a very fun technique to explore areas you were otherwise prohibited from exploring… think of it like climbing the Mount Everest, few people have done it, and the view is spectacular. Although it could also be used to kill people without them having any way to fight back at you unless they too were ranged.
Pathfinding bugs which meant you could attack a mob and he would either be unable to reach you, or have a long detour, allowing you to kill it before it could even hit you – great way to level quickly.
Probably a few more I missed…
Fishing bots were very popular, since it is tedious and takes forever. The more advanced bots would either “scan” a designated area on the screen or have some other method of detecting the ‘splash’ when you are supposed to right click to get your reward.
Unfortunately a few of those bots etc have caused blizzard to remove the UI and game functions that had other usages except for botting, which makes the whole game suffer for it. :-(