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.
By far one of the greatest engines out there. id Software really has some masterminds working for them creating the greatest development kits out there.