I recommend using MP4 and WebM.
MP4 is supported by all major browsers and WebM is set to become the new defacto in the coming years I hope.

If you use MP4 and an HTML5 player (with a fallback to a flash player for unsupported browsers), then your videos will be streamable to mobile devices as well, something lacking from current file hosting sites with video streaming. In addition MP4 files become relatively small when converted from avi's and such. Example I convert a video on my desktop from a 1.5GB avi to a 121MB MP4.