Multi-Theft Auto, the multiplayer deathmatch patch for the PC release of GTA3, has reached the mindbending milestone of version 0.3b (well, you know), and is available here sized 4.9MB. The authors encourage players to download the PDF-based manual too.
Having recently tried GTA3 multiplayer, we can confirm that it works, but we can't really say much more than that. We found our fellow players all of three times in an hour. It seems that it's more fun arranging impromptu races, stunt competitions and generally co-operating with one another than anything else, because the sprawling Liberty City never worked as a deathmatch arena and it doesn't now. Which is no reflection on the makers of MTA - it's just a sad fact.