Shadow Complex was a fun demo. That said, I still have Metroid: Fusion to play from the Ambassador-program, so that'll be my next Metroidvania-kick when I get to it.
The Sposion-series is neat. I'm not crazy-impressed by those games, but they do what they do well. Do they stay fresh as you get deeper into the games?
I'm not too familiar with the rest so far (Shank's, Fex, Skullgirls, Dishwasher 2), so I'll look for their demos soon.
My friend has also begged me to buy ilomilo, which seems to be maybe the cutest game I've seen in a long time. Its demo was quality too, so it may be a nice contrast after finishing Bioshock!
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http://img812.imageshack.us/img812/3492/eponaharr.jpg
Nominate SuperNiceDog's Guru-choice, then Epona.