1 craigslist and a few ebay deals. Craigslist was a great deal on a tri bike. I got the impression it was a bad breakup, because the 5'10" guy selling a bike for a 5'3" person also threw in 2 pairs of women's shoes with cleats with the bike. I think he just wanted the stuff gone.

Ebay - my previous and current road bike. Previous road bike was a great deal, but didn't end up fitting quite the way I needed it to. I still rode it for a year and sold it to a friend who loves it. I got many compliments (and offers) on that bike.

Current bike I bought the frame/fork and gruppo (SRAM Rival) on ebay. Rest of the parts at the LBS. I love love love this bike! It was a good deal (but not outstanding). The parts were a great deal though - and from an authorized dealer so I still have warranty on them.

I look for listings with lots and lots of pictures, and I'm not afraid to ask questions. I also like to see some reputation, although the higher the reputation score, the more likely they will put a high reserve on it. The best deals come from folks who don't really know what they have...