I've used a lot of different desktop software packages for diagramming software - some I like, some I don't, but most of them are relatively the same. Unfortunately, I've never found a web-based diagramming tool I like... until now.

A coworker recently introduced me to gliffy (http://www.gliffy.com/) - a Flash based online diagramming application that's very simple to use but is pretty robust. Oh, and it's free, too.