[simonlc]

There are a ton of quake clients, some that are very minimal (and just try to fix bugs and support new machines), and some that add or enhance new features or engine effects. There are also some that are built with competitive multiplayer in mind. My two favorite are Fitzquake and Fisheye Quake.

Also you might want to check out http://celephais.net/board/news.php which has a ton of very high quality user content for additional chapters, like mini expansion packs. Some really great level designers and artist who are making still making stuff for this game.