Open-source software is a beautiful thing. It's free, community driven, and more often than not, incredibly useful. HandBrake, one of the best video converters around, is a classic example of great open-source software.
The Swiss army knife of video converters and a perennial TUAW favorite, HandBrake is a Mac users best friend. Using libavformat and libacvodec, it'll take almost any video format or container you want to throw at it and convert it into something more useful. Want to convert a video for playback on an Apple TV? No problem. Open the offending video file, select "AppleTV" (version 1 or 2 depending on your model) from the preset list under "Apple" and click the "Start" button.
If you want to get a bit more involved than simply using presets you can tweak almost every video and audio conversion setting available. From bitrate, FPS and target size, to video and audio codec, embedding of subtitles and breaking-down the video into chapters, you can do it all with HandBrake.
HandBrake is available for the Mac, Windows and Linux, but on a Mac it's 64-bit, multithreaded and will use all of the available resources of your modern Intel-based machine. That means conversions are fast and efficient. If you've got a video file and you need something to covert it then HandBrake is the tool you need. It won't decrypt things, so if you're trying to rip a DVD or something similar, you'll need a tool to remove the copy protection first, but that video of your mate jumping the gate you shot won't know what hit it.
Download HandBrake for free from the HandBrake Project website.