I'm interested in knowing what kind of game tweaks are considered legal/safe and what kinds are considered cheating.

Gun Textures?
Gun Models?
Player Skins?
Sound Tweaks?
GUI Teawks?
Effect Tweaks?
Scripts?

It seems to me there are numerous ways in which a player can give himself an advantage over others, even simple crosshair adjustments can give players huge gains, and believe me, I know about huge gains