The last post linked to versions of an HD test video uploaded to YouTube and Vimeo. Video hosting services don't stream your original video, but re-encode to fit their particular display formats. So quality can vary quite bit depending upon the specifications of the original video you upload and the particular way a service renders into their final format.
Vimeo produced the best image of the two services I tried yesterday and was the least picky about image format. Today, I uploaded to blip.tv, and in my opinion it's the all around best choice. The image quality is as good as Vimeo, plus it's the only option that supports embeddable HD video (although I don't have the width available here for the full resolution.)