I have a program called SUPER and it can convert any kind of media file into a whole list of different formats. If you have Media Center connected to your 360 then this would be a good choice for some of you.
The best thing, it is absolutely free.
I'm not lying either.
The file is on the very bottom of the page and it says download SUPER setup file. I actually made it easier to find the download because if you go to google and search for it, go to the website, you then have to go through 3 pages of reading to find it.
Edit: Ok, nevermind, when you open that link you have to go to the bottom of that page and try to find it from there. I just tried it and it goes to the homepage of the program instead of the download page.