I have a 2TB hard drive (that's formatted for a Mac right now) that I have hooked up to a hacked Apple TV, which acts as a file server/media center. However, my PS3 can't seem to find my Apple TV, and thus can't play the files that are on the hard drive.

Can I, instead, hook that hard drive up to a PS3 and have the PS3 play the files that are on it?

Furthermore, can the PS3 play DivX/XviD/whatever movies, or anything?

Yes, you can connect external HDDs to your ps3. Make sure you format it to Fat32. And yes, it will play DviX. Here's a list of all supported video formats:

http://manuals.playstation.net/document/en/ps3/3_15/video/filetypes.html