I've been using Mambo/Joomla for some Non Profit Sites... Some of them are fully completed design wise, others simple usage of free templates with minor modifications, depending on the time and materials I had for that specific site...
Open Society Foundation Romania (not my design, just my implementation)
http://www.osf.ro
The Romanian National Coalition on Violence Against Women (mambo template)
http://www.nuviolenta.ro/
The Novell/SUSE Linux Users Group Romania (mambo template)
http://www.suse-linux.ro
The IT Solution Magazine (IT Management Magazine - original design, not mine, but my implementation)
http://www.solutia-it.ro
Partnership for Equality Center (not my design/implementation, but I maintain the site)
http://www.cpe.ro/romana
and there are a few others on the pipeline...
Daniel