"what it does is provide a logging utility like Firebug’s console.log that properly inspects objects and shows you it’s members. If you log errors it formats them nicely too. If you inspect DOM elements with it, it attempts to show you where on the page they are and calculate their widths and heights and positions on the page. You can resize and drag NitobiBug around the page, and it tries to remember where you put it."

Here's a quick tour and screencast:

It's free and open source so go download it.

[Disclaimer: I work with Alexei at Nitobi, in fact I sit right next to him:)]