Mark Harrison, CMS Guru extraordinaire in the UK, has started another site dedicated to compatible solutions to extend Microsoft Content Management Server (MCMS) and SharePoint Portal Server. This is the MCMS Plus site. Excellent work and a great place to see all of the extensions in one place.
Also, make sure that you take a look at MCMSFAQ.com. Spence does a great job with this one. He has recently changed the name to MCMSFAQ.com, so please update your links.