If you have the game already and want some fun (or if you are simply crap at it) then you might want to check out this list of confirmed cheat codes. All cheats have been tested successfully on the Xbox 360 version of the game.
All cheats should be entered during normal gameplay using Niko's cellphone. Enjoy.
In other news, the GTA forums have been expanded to include several new forums, so the chaos has spread a bit and is now a little easier for us to manage. Remember that registering is free.