The do-it-yourself Raspberry Pi boasts some pretty modest specs, but it's been proven time and time again that it's no slouch where gaming's concerned. In November, it was announced that Minecraft is being ported to the platform (it's still not here as of the time of this post), and not long after, the Raspberry Pi App Store was rolled-out offering a couple of open-source games that run just fine on the platform.
OpenArena will look instantly recognizable to anyone who's ever played Quake III, as it's based on the open-source port of the id Tech 3 engine, ioquake3. Development has been on-going since 2005, though the game is still not considered "final" software. However, it's still very well-developed at this point, so it's well worth checking out. Future developments may include things like map packs, not necessarily engine enhancements.
It's worth nothing that OpenArena is a very popular inclusion for most Linux repositories. It's currently available in Ubuntu, Fedora and Gentoo, and likely many others.
With a game like OpenArena making its way to the RPi, it doesn't seem unlikely that we'll soon see other ioquake3 and Quake-based games hitting the platform in the near-future. Other key ioquake3 games are Urban Terror and Tremulous, with other games that should work on the RPi being Nexuiz (the original) and Warsaw.
How cool would it be to have a LAN party sporting RPis instead of desktop PCs?