Whether you're looking to change a video format to save space or because you lack a necessary codec, the freeware FormatFactory could be the last converter you'll need. It's not perfect, but it handles 12 types of video formats, six audio types, eight image formats, and DVD/ISO conversions with speed and accuracy.
The interface is simple and utilitarian, although spiked with format-icons and a big banner announcing the program's name for no reason other than to put lipstick on the pig. The left navigation menu has collapsible links to the various formats you can convert to, based on type: video, audio, image, mobile device, and ROM device. Below the massive title banner, the central pane sports decorative icons until you start the conversion process.
Once you've selected your files, you can change the output quality to presets or custom settings. Hit Start on the Toolbar of the main window to begin the process, which is surprisingly fast for smaller video files. FormatFactory excels at batch conversion without error. It does, however, eat up much of your processor while it's running even one conversion.
Because there are so many file formats out there and even more codecs, it's likely that FormatFactory will fail to convert at some point. Nevertheless, FormatFactory is a solid, free, catch-all converter, with excellent tools--lacking only a more sensible interface.