Rui CarmoOpenArena is an open-source content package for the id Tech 3 source licensed under the GPL, effectively creating a free stand-alone game that is fast and true to the physics of the original.
It can be played at quite a fair clip on even low-powered machines (by today’s standards) like the Eee 901, and its only irritating flaw is the rather poorly designed character models (easily fixable via seta cg_forceModel "1"). Another (minor) flaw is the rather prosaic level design for the built-in levels (i.e., not quite up to Q3 standards).
Other than that, it is a great way to let off some steam on the Mac (where it runs swimmingly), Windows or Linux (where it will behave rather poorly if you have compiz enabled – the FAQ helps here.).