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.
We installed the trial version onto Confluence the other day and played about with it for the morning, fantastic bit of software.