Grand Theft Auto United is a modification based on 'GTA San Andreas' for the PC. The target of this modification is to unite in only one game the game areas, the so called 'maps', of the two GTA Games 'GTA III' and 'GTA Vice City' on basis of the graphic- and physic engine of 'GTA San Andreas'. The map of 'GTA San Andreas' therefore gets replaced in the current version.
I had quite a lot of fun with 1.1 even though it was a bit buggy, but that's not so bad as long as the game doesn't keep crashing doing anything like some games on Steam do, which this mod clearly didn't. In overall a really great mod and surely worth trying, especially now that 1.2 is out (which I haven't tested yet).