AssaultCube is a FREE, multiplayer, first-person shooter game, based on the CUBE engine by the team at Rabid Viper Productions. Taking place in realistic environments, with fast, arcade gameplay, it's addictive and fun!
With efficient bandwidth usage, it's low-latency and can even run over a 56Kbps connection. It's tiny too, weighing in at a lightweight 40 MB package available for Windows, Mac and Linux. On the correct settings, it can even run on old hardware (Pentium III and above).
You can read more about AssaultCube from the website @ http://assault.cubers.net/
If you're curious about how to participate in the community, then take a look here: http://assault.cubers.net/community.html
If you need some help with anything at all, please first search through this official documentation. If you still can't find the answer you need, search the wiki, and the forums, and if you still can't find the answer, post a thread on the forum explaining your situation. You can use this search engine to search the whole lot in one go!
You may also try IRC for a real-time response, but be patient (at least 30mins), as people "idle" in IRC. You might get an instant response, or it may take a little bit of time. Just hang out and see!