It's a really easy-to-use drag-and-drop tool that makes it simple to create mockups. To be fair, the functionality and features are limited but it's free and it's incredibly easy to share your plans.
Here's one I knocked up in half an hour:
University wireframe webpages created using Mockingbird
- It's not a patch on Axure - but it's free and requires no software other than your browser.
- It's more limited than Visio - but it's online and incredibly easy to distribute your work.
As it's currently a beta, I expect it will continue to improve, and no doubt at some point it will cost.
But for now, it's great value and in some cases it's the best option if you need to prototype.
If you're planning and developing websites and applications set up an account. There's no excuse not to!
Mockingbird free online prototyping tool - www.gomockingbird.com
Update: July 2010 - Mockingbird service enhancements