Many of the bikes in this thread don't fit my idea of a beater bike, but I guess that's a relative term:
Show Us Your Beater Bikes!
One of my winter projects this year is a Box-O-Crap build of a 1973 Nishiki I bought as a donor bike for a Clunker 100 Challenge a couple of years ago. That's right, this bike was so bad I decided to use it as a parts bike for a Steyr Clubman build. I bought it for $25 from a guy who told me (while we were negotiating the price!) that he was going to send it to the recycler if I didn't buy it. It's heavy and lousy with rust, but I think it has potential.
I also have a bit of a weakness for frames that have been badly abused. I bought this 1980 Schwinn Voyageur frameset for $20 (probably overpaid a bit). It had a twisted and bent fork and bulging top tube and down tube. After a visit to Atelier Gugie and a bit of hastily applied hobby paint, it was ready for the road again.
This one might be worth stealing for the parts -- in fact it's currently hanging from a hook in my garage stripped of most of the parts shown here -- but the frame itself is definitely a theft deterrent. It doesn't look nearly as nice in person as it does in this picture.