CastCluster is an open source project that I wrote specifically for converting YouTube, Google Video, and local files to a format that I could play on my Apple TV.
[robg adds: With the apparent popularity of the Apple TV, apps like this one will prove useful to many folks -- if you have a favored way of converting such video, please list it in the comments.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070411081156349