The famous SA:MP! Few have heard of him yet, and if you haven't, what's stopping you from getting to know him?
This modification requires an installed (preferably clean) GTA SA game and allows you to play online.
The archive contains a complete set: the SA:MP 0.3c R3 client itself, the 0.3c R2 server for SA:MP scripters, a small manual on navigating in the SA:MP window and instructions on how to use the "+c" bug!
When installing SA:MP, specify the folder with GTA SA (I have "D:GamesRockstar GamesGTA San Andreas"). After installation, the "samp.exe" file will appear in the game folder, and you need to run it.
To run game, in the SA:MP window, click on the green square in the upper left corner. If after clicking on "Connect" you open a regular game, then replace gta_sa.exe with one of those presented on LibertyCity.ru. Should help.