Smart Converter

SmartConverter [app store link] is a FREE transcoder that outputs to many popular formats.

It has some good reviews and could be handy for quickie conversions.

It'll transcode (sometimes just re-wrap which is really fast) to:

Apple TV
Google TV
iPod
iPhone
iPad
PSP
Playstation 3
Sony Barvia TV
Xbox 360
Windows
Android Tablet
Android Phone
Music (just audio)

It's super simple and you can click on the thumbnail to get some info about the video you want to convert.

It'll tell you what it thinks the transcoding speed will be in terms of Slowest > Fastest (if no transcoding is needed, just a re-wrap).

I didn't try to see what happens if you drag less common formats into it without Perian installed but with it installed it seemed to like an .avi (mpeg4 with mp3 audio), an .mkv (H.264), and a random .flv. It even accepted a .FLAC file.

After converting it automatically "sent" the audio file to iTunes but displays a "Show File" button which is handy because it kind of buried the transcoded audio file in ~Movies/SmartConverter/ which I probably wouldn't have found right away on my own.