The IE team has released a developers toolbar for IE that makes a lot of nifty features available for developers.

The story is on the IEBlog website and the download is available as well.

Some features that I really like
- DOM explorer (as a pinnable toolbar inside IE)
- Highlighting of divs, tables, etc
- Inbuilt ruler for measuring distances
- Preconfigured windows resizing option

Really nice stuff :-)