When I tried to convert some of my old .wmv video files on my Mac, I didn't find any easy and free solution. There is a lot of video conversion software for Mac around the Interwebs, some of those apps are quite good - but there's one thing they've all got in common: They cost money. So I continued searching and spent hours till I found my personal free solution: ffmpeg. It is extremely powerful, and free! And it is a Shell-based conversion tool.
In general, I love to work with the shell. But it can get really exhausting, and it got exhausting in this special case. So I decided to create a very small AppleScript, also available as a Droplet-App for Finder. Thus you just need to drag the desired file onto the script/droplet, and it will do the job for you. Quite comfortable, I think.
So what do you need? I'd recommend you to get Homebrew, my favourite package manager for mac: http://github.com/mxcl/homebrew. Once installed, just type "brew install ffmpeg" to let Homebrew do the complete install of ffmpeg for you.
Now that ffmpeg is installed on your system, feel free to grab my very very very simple ffmpeg converter Droplet / AppleScript. Extract the package and drag the droplet ont your Finder bar. To convert any video to a hq mp4, just drag and drop the file onto the droplet icon in your finder menubar. Have fun!
Download: ffmpeg Applescript & Droplet v0.1