In the past for media conversions I always like Pazera's free converters. The problem with Pazera's converters is that there is a separate converter for each format. If you want to convert AVI to MP4, you have to download one converter, if you want to go from FLV to MPEG, that's another one. It adds up fast if you do a lot of media conversions.
I found one today while looking for a free MP4 to FLV converter. It's called Quick Media Converter HD or QMC. Like just about every media converter out there it is a GUI front end for FFMPEG. This tool is really easy to use, and you don't need to be a super genius to click the buttons and convert files.
Here is a screen shot of it so you can get an idea of what you get with it:
As you can see from the picture, it's really intuitive. The icons across the top in Easy Mode really does make it easy to use. One thing I should tell you is that this software is packaged with a tool bar that you can opt-out of at insstall time. Make sure you watch for it though, and don't just keep hitting Next to do the install.
What's your favorite media converter? Do you use something else? Let us know in the comments!