Urban Terror, a free ioQuake based online multiplayer FPS. It requires only a 25 k/s connection, so it's perfect for when my family hogs the connection,and it can run on my old linux box(1.2 ghz, 256mb RAM, Integrated Graphics) and my laptop(1.6 ghz intel celeron, 512mb RAM, unknown discreet graphics), and still looks good on my 20' monitor. It also can be run from a USB stick, and only takes up ~900mb of storage.
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Guns don't kill people--Magic Missiles Do.