I realize this one was already on Steam Holiday Sale for $7.50 but today Batman Arkham City: GOTY is at $6 on GMG (with this code GMG20-PJFEW-Y16HK) – can be activated on Steam but installs a bunch of other DRM stuff along with it.
Sword of the Stars II: Enhanced Edition is also on sale right now and with the 20% voucher (GMG20-PJFEW-Y16HK) makes it only $4 – the game is also redeemable on Steam.
Agricultural Simulator 2013 (really? just name it FARMING 2013 :D) is at 50% off on GamersGate which makes it around 10€ – depending on region. I have no idea if this one activates on Steam.