Whether you're looking to change a video format to save space or because you lack a necessary codec, the freeware FormatFactory is likely to 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 nav holds 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 the aforementioned decorative icons until you start the conversion process.
To do so, click on the icon on the left of the format you'd like to convert to, such as All to WMV. From the right side of the new window that opens, you can add files one at a time, or choose to convert an entire folder or three. The batch conversion feature is one of the app's strong suits, and it handled multiple files for me without error. Unfortunately, drag-and-drop file adding isn't supported.
Once you've selected your files, you can change the output quality using the Output Setting button at the top of the "All to" window. The Output main window informs you of the current output format details--if it's a video, for example, it'll tell you the output codecs being used and resolution. These can be changed via the Profile drop-down menu, but they change depending on the output format. Still, all output formats offer custom settings, another excellent feature for users who get feisty about how files are encoded.
When you're ready to convert, hit Start on the Toolbar of the main window. I'd normally suggest going to get a coffee at this point, but unless you're re-encoding an entire movie FormatFactory flies through a task like a dedicated assembly-line worker. For larger files, the Factory naturally takes longer. Either way, it will most likely demand the lion's share of your system's resources while it's running--it just does it so fast for smaller files that the wait can be stunningly short.
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. It succeeded with one Quicktime file I had, but couldn't properly convert a different MOV. Nevertheless, FormatFactory is a solid, free catch-all converter, with excellent tools like batch conversions, lacking only a more sensible workflow.