Star Wars: Clone Wars
Not to be confused with Star Wars: The Clone Wars (either the film or the TV series), this is the 2D-animated series that originally bridged the gap between Episodes II and III.
Originally released as 20 three-minute shorts across two seasons (though it's clearly one work -- presumably there was a gap halfway for production reasons), on DVD it's edited together into an hour-long movie. Its origins show through in just how action-packed and fast-moving it is, though that's no bad thing. The story is relatively slight, but it's beautifully animated, put together with a great amount of skill and creativity.
For the really interested, there's a ludicrously thorough episode guide (as in, each of the three-minute segments has its own entire page) available on Wookieepedia -- here's season one (chapters 1-10), and season two (chapters 11-20). All things considered, there's a surprising amount of detail there.