If you just want to play multiplayer, get this!
Download the SA-MP client installer from one mirror and install the program in to your GTA San Andreas folder.
You can use Open.mp Launcher as an alternative to SA-MP Client (you still need SA-MP installed though).
Note: This package also includes the scripting tools (PAWN compiler).
Please review the SA-MP Services Agreement if you wish to run a public SA-MP server.
Open.mp is fully compatible with SA-MP server!