Pencil is a cross-platform animation creator that uses the old-fashioned, cell-by-cell process to create cartoon quality movie clips. Movies can have both bitmap and vector images and are composed in layers, which allow for some nifty effects. Completed movies can be exported to a variety of formats, including .swf and .mov.
I have been experimenting with Pencil for several weeks and will be introducing it to my 8th graders when we return to school after our December break. Hopefully I will have examples to post here soon!