If you use iTunes to organise your media, you've probably run into the roadblock of adding files it doesn't support (like MKVs). Here's how to trick iTunes into support the files you want.
The trick works by changing the file type flag in the file. You can do this with a utility like FileBuddy (Mac OS X) and just change the file type (for AVIs it seems to be VfW) to something iTunes can support (like MooV). Save the file and you'll suddenly be able to import it into iTunes. To play it back, however, you'll need the proper codec support. Perian handles this brilliantly on the Mac.
For Mac users, there's an even faster way. MacRumors forum poster Chris14 created an AppleScript application that automates the process (download it here). To use it, just open it up and select the file you want. The AppleScript app will change the file type and you'll then be able to import it into iTunes.
On Windows, you'll need to do a little extra work and you'll need QuickTime Pro. If QuickTime can interpret the file, you're in good shape. Just save the file out as a QuickTime reference movie and add that to iTunes. To help iTunes open more difficult formats, try installing additional video codecs to add support. 3iVX can help with DiVX-encoded files and XiphQT for a few other file types.
How to make iTunes accept other video formats [MacRumors Forums]