On the riff trax site they have a place where you can make your own voiceover riffs to pre-selected clips. One of the clips is funny without voiceovers..check out the clip "how to treat loose women". I can't link the clip directly but I can link the page just scroll down to find it.
http://www.rifftrax.com/cuts/videos
I thought it was an old PSA for the Military. But its much better.