QServer: Website Server Browser Plus In-Client Server Browser Feed
License: GPL --- you know, Open Source
Download: http://servers.quakeone.com/QSBrowser.zip
Example: http://servers.quakeone.com
Requires: ASP.NET and SQLite (may be other requirements)
Target Audience: Anyone who wants to make a total conversion Quake game who would like to have a nice portal web site kind of like QuakeOne.com and wants servers list with players and an in-client server browser feed.
Example Pic -- example looks a little vanilla, but obviously you can customize it. Apparently uses AJAX and updates on-screen.
Features: Add servers, administration controls and more.
Plus it can feed a prototype server browser for ProQuake that I will be tutorializing for the addition to any engine.

(Yes, DarkPlaces sv_heartbeat and master server is a superior way and one day hopefully that will available in standard Quake, but this works with all standard Quake server today. Sometimes you have to work within the framework of what can be done today.)