MediaCoder is a free do-everything, convert-everything audio and video batch processor. It relies on tools like ffmpeg behind the scenes, but supports multiple engines, lots of formats, and has a graphical front end. It works on Windows, and could be a good reason to virtualize Windows on Mac or Linux, and it also evidently works pretty well on Mac and Linux via WINE. (Haven’t tried that yet; see the download page for details.)
The best news from MediaCoder land is that a recent build has added support for AVCHD, the widely-used HD format. This is essential for those times you get media off someone’s hard drive-based player. The MediaCoder folks have a brief tutorial with screenshots on their site:
If you have a preferred conversion method for AVCHD or other formats on your platform of choice, let us know. In the meantime, I’m finding I fire up MediaCoder almost every day.