One tool, VirtualDub, can be found at http://www.virtualdub.org/

Another tool would be the Windows Media Encoder:
http://www.microsoft.com/windows/windowsmedia/9series/encoder/default.aspx

Blaze MediaConvert is one that I use and it is a good and simple converter - - http://www.mystikmedia.com/

For a more robust product that also allows you to edit, etc., I recommend Blaze Media Pro at http://www.blazemp.com/blaze_media_pro.aspn