FFMPEG GUI v1.1
I made this program for users like me that need to have a video converted once in a while, be it a flv file downloaded from youtube or an avi downloaded somewhere.
This program can convert and extract almost anything you can throw at it. It can convert between avi, mp4, mpg, wmv, etc. and extract audio in mp3, ogg, wma, etc.
Just try it yourself, it's not hard at all.
Note: This an application for Windows written in C#, so it please beware you'll need the .NET framework v3.5 to run this.