Free or commercial? I use Sony Vegas Pro which is a full-featured editor, some people prefer Adobe Premier. There are basic versions of each program that are about $50 which will do the trick. I'd suggest downloading a trial version and seeing which one is easiest for you to use. You can find the Sony products here:
http://www.sonycreativesoftware.com/download/trials but the Adobe ones you'll have to Google for version 4 unless you have an up to date computer. The newest version of Premier only runs on a 64-bit operating system. For the Sony programs, Vegas Pro is the big one, Movie Studio HD is the $50 one, and the Movie Studio Platinum versions are probably around $70-$80. I'm not sure about any free programs, but there are probably some out there. You can search this site to get more ideas.
http://www.videohelp.com/tools