(Note: This is from 2001 and has not been touched since)
mp1e is a real-time MPEG-1 video encoder that you can use to create MPEG-1 files from your bttv or video4linux2 device!
Unfortunately, it only creates I frames, and the MPEG-1 streams internally look more like MPEG-2 so all the Windows players that I tested crashed when trying to play those streams. But mtvp can play them under Linux.
mp1e requires an MMX cpu. I didn't write mp1e, the above link is to my knowledge the original location to get mp1e from, but if you are using a recent gcc or egcs version, the stock distribution won't compile.
I also offer a patch to enable input switching so you can record from S-Video on your bttv card, although that is really a crude patch. YMMV.