Bzflag

Used by 37 people for 124 hours, 38 minutes and 52 seconds

BZFlag is a free online multiplayer cross-platform open source 3D tank battle game. The name originates from "Battle Zone capture the Flag". It runs on Irix, Linux, *BSD, Windows, Mac OS X, and many other platforms. It's one of the most popular games ever on Silicon Graphics machines and continues to be developed and improved to this day. It's one of the most popular open source games ever.

There are over 200 servers on most the time. The maps get created by the users, some use a WISYWIGs or they code.