After listening to the most recent Three Moves Ahead, I took interest in Mode 7’s beta of Frozen Synapse, a turn-based tactical game with the aesthetic appeal of Introversion’s Defcon and the tactical gameplay of X-COM or, as Rob Zacny described it, a paper version of the original Rainbow Six—all of which gets my horse racing.
Incidentally, Mode 7 released the Mac beta today. You must pay for the beta, but in return you receive a secondary beta license for a friend and a copy of the retail version upon its release, which Mode 7 says will be late 2010.

Pay for an unfinished product? Ah, delusional developers…
Well, I think the idea is to pay for a product that is stable and playable but still unpolished. I say this without having played much of the beta, so my opinion is without experience.
Mount and Blade did a similar thing, asking users to pay for the beta before the final release. It wasn’t required, though.