First, from the ashes of sa-mp comes a brand new multiplayer mod, GTAMultiplayer. They have just released a video showing the excellent mod, and say that it is around 75% finished.
Visit the official GTAMultiplayer site Here for more information. You can also download the video in the media area.
Also, another mod called GTA Rumble has been released, and is played by many members of our forums. It is an excellent multiplayer mod with many vehicles and is very smooth if you can find a decent low ping server. Download the mod from the Rumble Website, in the downloads section.