Maybe you know it, maybe you don't but yUML is awesome!
Create and share simple UML diagrams in your blogs, wikis, forums, bug-trackers and emails.
Yesterday I was creating a new class diagram using yUML and besides it's a great product it was a bit cumbersome working in a small textarea and pressing 'Generate!' (all the time). I fired up textmate and created a diagram using the yUML syntax. A couple of minutes later I had ruby script which generated the diagram using yUML by taking a text file as input.
sudo gem install yumlcmd
yumlme -f your-diagram.txt
Syntax overview: http://yuml.me/diagram/scruffy/class/draw
I hope you like it as much as I do :)