iFFmpeg is a free, easy-to-use front-end for the powerful--and also free--command line tool FFmpeg, which lets you easily convert media files between different formats. (Note that you must also download an FFmpeg binary to use iFFmpeg, but the developer's Web site has simple, drag-and-drop instructions for doing so.)
The iFFmpeg interface is straightforward enough for even beginners to use, with a sparse menu bar that lets you add a file to convert, or preview, start, or merge files. You can also look at FFmpeg binary information, choose a codec (from an almost overwhelming array, from basics like AVI, MOV, and WMV to device-specific MP4s for everything from AppleTV to Sony PSP), and adjust a huge number of settings, include bit rate, resolution, and aspect ratio, with separate settings for audio and video. iFFmpeg can also handle batch processing and comes with several localizations.
The latest version improves option help tags and iOS device presets, along with several other feature tweaks and fixes.
Add to all this a responsive developer and a happy user community, and iFFmpeg is a must-download for anyone who wants a free, simple media converter.