OK, I confess. When I see an old bicycle and/or parts thrown out in the alley for the trash, I find myself drawn in. At first, it was just to get salvageable parts -- brake pads that still work well, seats in good conditions, pedals that accept toe clips, etc. But as I slowly build up my knowledge of bicycle maintenance, I find the process to be educational. For example, if I have to do a type of repair that I've never done before, I like to use a junk bike from the alley for practice. I also like the fact that tinkering around with the alley bikes lets me get hands-on experience with differences between bikes.
So far, it's just been components I've been salvaging, since the frames often tend to be rusty or too small, etc. I know someone whose favorite bike (if somewhat a quirky bike) was salvaged from the trash, but I know that those sort of finds don't happen everday.