Direct2Drive's "Deal of the Week" is
Defense Grid: The Awakening
at 50% off. That's $9.99 and since Direct2Drive has yet to screw over Europe like other digital distributors did, it's a fair €7.70 + VAT for EU citizens.
What would be interesting to know before purchase:
I thought I read that the Direct2Drive version also could be / had to be activated onto Steam.
Was that and is that still true? I find no mention of Steam on Direct2Drive's Store page for Defense Grid - other than in the form of "Achievements" but there are several libraries in place to offer Achievements and stats independent from Steam.