Pre-order BioShock Infinite on Steam for TF2 hats, free games; Amazon offers $30
Pre-order BioShock Infinite on Steam, and you might get free copies of BioShock and XCOM: Enemy Unknown. In a Ninostarter-type drive, rewards are unlocked as the number of pre-orders goes up.
The first tier is a free copy of BioShock for everyone; next is a set of BioShock Infinite-themed hats, because it is a game on Steam. Finally, if pre-orders reach a high enough number, free copies of XCOM will be rewarded to everyone.
Amazon offers $30 in "2K Games Credit" with pre-orders of the PC download. You could always fold up some dollar bills into a hat.
