I've been a beta-testing Autodesk's Sketchbook Pro for years now (since version 2), so I'm really excited to see the new version (2010) is getting released soon. Sketchbook is a lightweight, fast, digital sketching software that I use every day in my workflow. If you've never tried it, you owe it to yourself to download the free trial here.
There's also a great article outlining some of the new features coming out in the 2010 release on Autodesk's AREA. The ellipse and rulers have changed my life.
This is really a great product that not a lot of artists and designers know about, and that's a shame. So check it out. It's cheap, it's great, and you'll never go back to sketching in Photoshop.