I'm continuing to play with stop-motion, and ran across this superb free [edit: it's not technically open source] pencil test program. To save myself some time, here's the blurb about on the Monkey Jam website:
MonkeyJam is a digital penciltest program. It is designed to let you capture images from a webcam, camcorder, or scanner and assemble them as separate frames of an animation. You can also import images and sound files already on your computer. Although it is designed for pencil and paper, MonkeyJam can also be used for StopMotion animation and has several features just for that. Movies created in MonkeyJam can be exported as AVI files."
Best thing about it so far is the X-sheet. For you 3D animation-only dudes, it's very similar to Maya's dopesheet (properly speaking, it's the other way around). Easily move your drawings around and change timing. It's great!
Only drawbacks so far are the lack of onion skinning and a grid overlay like StopMojo has. But then the latter doesn't have the x-sheet.
I'm sure there are some better programs for money, but this one is free. I plan to make a tutorial with it soon (emphasis on PLAN).