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If you sign up for a 30 day trial for IGN Prime, you can get a free copy of Orcs Must Die!. You can also get DLCs free for Sanctum.
These are both tower defense types of games. Registers on Steam.
Oh also as an IGN Prime member you get 15% off purchases at Direct2Drive. Make sure to disable the auto-renew option when you sign up. You can cancel right away and your membership stays active for the 30 days so you can still take advantage of the 15% off. Might be a good way to pick up Skyrim for a good price.
UPDATE: They’ve ran out of CD Keys for Orcs Must Die! Sanctum DLC is still active however.
doesn’t work anymore ;(