The Disappearance of Haruhi Suzumiya and
Trigun: Badlands Rumble are good, but you need to see their parent anime to appreciate them. Which you should do anyways if you haven't.
Eve no Jikan Gekijouban is good, though basically a compilation of the web series. I'm actually not certain which version is more complete. I seem to remember there are scenes in the film not present in the series...