There are a lot of very expensive UML tools out there, that promise the world. They'll integrate with your SCM system and write half your app for you, AND round trip changes back into your design. They'll make nice diagrams and you'll get to join the 30,000 foot high club.
The problem with these tools is that 99% of the time, you just need something to jot some ideas down to convey an idea or get your thoughts straight. So, rather than learning Enterprise Architect, why not try one of the following
Sequence diagrams - http://www.websequencediagrams.com/
Using simple text markup, you can produce sequence diagrams, complete with comments and flow control blocks. If you want cloud storage for your diagrams, they offer a subscription service, and the guy behind it is really responsive to questions and feature requests.
Flow charts and block diagrams - http://www.asciiflow.com/
ASCII art, with a point-and-click interface tailored to a boxes-and-arrows drawing style. And it's open-source, so you can run your own instance, if the fancy takes you.
(more tools to come, as I find them)