Without the RAD Video Tools, I'd be stuck with Quicktime movies instead of Xvid, DivX, and WMV files. Seriously, you can convert just about any kind of audio or video file with ease – and this sweet li'l tool also happens to be free. It's unobtrusive, relatively straightforward, and fast. For any kind of conversion, I'd suggest starting with the raddest freeware around:

The RAD Video Tools are a set of utilities for processing video, animation, and sound data. They feature both of our video codecs: Bink Video, our latest 24-bit true-color codec, and Smacker, our 8-bit 256-color codec.
Bink Video is a “better-than-DVD” video codec. That is, it compresses better than DVD at up to three times the playback speed! Bink is a hybrid block-transform and wavelet codec that can encode your video using 16 different compression techniques (wavelet, DCT, motion compensation, a variety of vector quantizers, Smacker-style, and more). With all of these techniques in one codec, Bink can handle pretty much any type of video. It also has a psycho-acoustic based audio codec that is capable of 8 to 1 perceptually lossless compression, so your audio will sound as good as your video looks!
Smacker is our 256-color compressor for video and animation. It has used in all aspects of multimedia application design: cinematics, cut-scenes, video-sprites, transparent videos, single-image decompression, scrolling video backgrounds, and more. Smacker has been used in over 2,600 games because it's fast, it's easy-to-license, it has a terrific SDK, and its video quality is unrivaled in 256 color mode.

I actually came across it when I needed to find an easy way to convert “MOV” to anything else a few months ago (when I got a stupid Nikon Coolpix 8800 that that didn't record to an AVI wrapper). It's worked like a charm!