QuakeForge is one of the many free Quake engines out there. Upstream hasn't packaged 0.5.5 for some reason, so here it is.
[ Packaged for: lenny (stable) | squeeze (testing) | sid (unstable) ]
Insert one of the following lines in /etc/apt/sources.list:
For the current stable Debian release (5.0) (lenny)
deb http://www.fbriere.net/debian/dists/lenny quakeforge/
For the current testing Debian release (6.0) (squeeze)
deb http://www.fbriere.net/debian/dists/squeeze quakeforge/
For the current unstable Debian release (sid)
deb http://www.fbriere.net/debian/dists/sid quakeforge/