It seems like someone whould have beaten me to this, but I went to a GameStop today to pick up Castlevania Legends on Gameboy (it was the last copy in a 200 mile radius; luckily the store that had it was about five miles from where I work), and they had a buy two used games (on any system, mix and match; also the 10% off card works in conjuction with the sale) get one free sale going on. I go to GameStops all the time and didn't know anything about this, so I thought that maybe it was exclusive to that one, since I had never been there before. But then, when I got home, I had an email from GameStop saying that the sale is at all stores and ends on Saturday. Huh.
Anyway, I got Castlevania Legends (GB), Castlevania Adventure (GB), and Mr. Mosquito (PS2) for $24.24 after tax.