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 (oldstable) | squeeze (stable) | wheezy (testing) | sid (unstable) ]
Insert one of the following lines in /etc/apt/sources.list:
For the current oldstable Debian release (lenny)
deb http://www.fbriere.net/debian lenny quakeforge
For the current stable Debian release (squeeze)
deb http://www.fbriere.net/debian squeeze quakeforge
For the current testing Debian release (wheezy)
deb http://www.fbriere.net/debian wheezy quakeforge
For the current unstable Debian release (sid)
deb http://www.fbriere.net/debian sid quakeforge