A picture's worth a thousand words, so here are full-quality screencaps from all five services, including a baseline of the source video in its native resolution. Since the various players either stretch or resize the source content to fit the player, black space has been added around the outside to balance. The services have been ordered from largest to smallest, starting with the source material.
Note: To compare, just mouse over the name of each service below the picture. It might take a second or two to load, depending on your connection.
For smaller-resolution stuff that hovers around VGA, you're better off with Veoh. Veoh manages to retain a good amount of detail in your video stream, as long as you're willing to put up with the lengthly uploading time. If you're lucky enough to be the proud owner of an HD-capable camcorder or digital camera, pass go and check out Vimeo, which is the only service that's ready, willing, and able to handle HD video, if you've got it. Now go forth and upload.
2/5 Update: Several users have noted the similarities in Veoh and Vimeo, and it's definitely a close call. I stand by my judgment of Veoh, as I've listed in the comments, but the difference will be negligible for most users. We're definitely planning on doing a follow-up with another crop of services, so if you've got a favorite that fits the specs for our test file, then leave it in the comments or shoot me an e-mail.
2/6 Update: I was lucky enough to get some time to talk about these tests on the News.com podcast with Erica Ogg earlier today. You can read the show notes and get links to the mentioned stories on this page, or listen to the entire thing with the embedded player below. If you like what you hear, you can also become a subscriber of the show via iTunes. One thing to note is I managed to mix up Vimeo with Viddler when talking about the two quality winners, so keep that in mind.