FlasKMPEG is a video conversion utility.
The basic idea behind it is that it takes a MPEG file (be it in MPEG1 or MPEG2), does whatever translations you want, and feeds it to encoder (or saves to AVI/OpenDML AVI file with desired codec configuration). You can use whatever video encoder you want. It tries to minimize the compression artifacts resulting in from multiple encodings.
Personally, I've used it somewhat to convert random MPEG clips that I've captured or downloaded off the 'net to VideoCD-compliant MPEG video streams, using bbMPEG as the encoder. So far I've only had one largely problematic file, all others have worked just fine. It does surprisingly good work - you can barely notice the quality degradation! (Or maybe I just need a professional-grade TV set to notice that =)
Many people, as said above, use FlasKMPEG for DVD ripping - of course, it has other uses besides that.
At the moment, it's only available for Windows, though. (Personally, I don't care - most of the working video capture/editing program I use are for that platform anyway =)
Home page: http://www.flaskmpeg.net/