ioquake3.org is the de-facto standard in Quake 3 technology post-opensource release. Others base themselves on the hard work of the hundreds who have contributed to ioquake3. Tremulous, World of Padman, Urban Terror. All use ioquake3.org for many reasons.
This is really advanced...
ioQuake3 Engine has so mani options than makes it "just wonderful"
As a start, you can play Quake 3 on 3D, that's just amazing. One of the reasons for why i buy red/blue 3D glasses.