QuakeTV
is a handy new piece of software that allows people to watch Quake 2 and Quake 3 Arena matches (almost) live over the internet.
It comes in three pieces - one streams a demo from the server the game is taking place on, another collects that stream and relays it to the viewers, and the third allows the rest of us to watch the game on a relay server. For a little more information, check the rather skimpy readme file, which I've posted here.
If it does what it says on the can, it should be well worth a look.