Otser's Site has the release of version 1.00 of QRec (pronounced cue-wreck), a utility that provides automatic unlimited demo recording for Quake, Quake II, and Quake III Arena. There is also news there on a bug in Q3A demo recording that may require a bit of working around:

There is a bug in v1.11 of Quake 3 that won't let you stop your recordings when a bot match has ended and the players are standing on their podiums. Apparently bound keys (like the one used to stop the recording) are not read by Quake 3 and thus your recording can't be stopped. The only workaround for now (until id Software fixes the bug, and I have E-Mailed them about it) is to lower the console and enter /stoprecord. Thanks to Barry Collins for this information.