Quicktime Player for Snow Leopard now allows you record the screen. Awesome!
Once you have a movie it will let you trim screen recordings for length.
Is there a way to crop the movie's dimensions, either in QT or using some built-in or free software?

Update: How to crop to an arbitrary size and aspect ratio? iMovie only seems to let you crop to the aspect ratio of the containing project.

Result: Both good answers but since I have QuickTime Player 7 and Photoshop, that's the workflow I choose as the answer. NOTE: If you have Photoshop Extended, you can import a movie, use the crop tool, and Export the cropped movie. Not free or built-in, but convenient.

To summarize the instructions from the video link ricbax posted:

1. Open movie in QuickTime Player 7
2. Copy a frame and Paste it into a new document in Photoshop
3. Draw a rectangular selection around the area to keep and fill with black
4. Invert the selection and fill with white
5. Save as .GIF, with 2 colors
6. Back in QuickTime Player 7, open Movie Properties window
7. Select the Video Track
8. Select the Visual Settings Tab
9. Drag and Drop the 2-color .GIF file onto the Mask drop area (or use choose file button)
10. Export the (now cropped) movie
    DONE