I stumbled upon MediaCoder while was looking for a way to move some music videos onto my iPod. That was over a year ago, and since then it's only gotten better.
What I love most about MediaCoder is the ability to take videos off YouTube or Google Videos and convert them to an iPod compatible format. Another, slighty more than amazing, feature is the ability to crop music and video to specific lengths. This has saved me TONS of money that I would have spent on downloading ringtones for my cell phone. I just load my MP3 into MediaCoder, select what section I want to split and hit start. 30 seconds later, viola, I have a ringtone.
There are some features that it's missing - where's the DVD converting capability? Where's the audio/video splicing? But it's free, and supported by an involved community, so I'm not going to argue. One warning - if your computer is a little low on memory, don't use this program. You, and your computer, will regret it.
I'm sure there are other uses for MediaCoder...I just haven't found a reason to use them yet. All I know is that it's made my iPodding experience that much better. Find a cool video? It's on my iPod. Buy a new CD? Yup, that's my ringtone. It's more than paid for it's free self.