1. FTEQuake

Website: www.fteqw.com
Documentation: www.fteqw.com/wiki/index.php

2. ezQuake

Website: http://ezquake.sourceforge.net
Documentation: http://ezquake.sourceforge.net/docs/#features

Other Port Clients:

1. FUHQuake

Website: www.fuhquake.net

2. ZQuake
3. Qrack
4. JoeQuake
5. MHQuake
6. TomazQuake
7. QuakeWorld
8. GLQuake
9. WinQuake
10. D3DQuake


=================
This is a Vote Only Thread
=================

Important Note: The item with the highest vote only represents people using the client. It does not mean that the client is superior to the other one.

2. ZQuake : http://zquake.frag.ru/

3. Qrack

4. JoeQuake : http://www.runecentral.com/joequake

5. MHQuake : Mhquake.com. This domain name expired on Mar 02, 2006

6. TomazQuake

7. QuakeWorld \
8. GLQuake > idsoftware.com?
9. WinQuake /

10. D3DQuake


maybe +ProQuake also?
it's far more popular then 50% of this list