logic/math based games?

[Dave Britten]

Spacechem is great. You constantly alternate between feeling like a gibbering moron and a complete genius while playing it.

Zachtronics also has a sort-of-sequel called Opus Magnum, which is played on a hex grid:

http://www.zachtronics.com/

And there's an iPhone game called The Sequence that reminds me a bit of Spacechem in that it has spatial/kinetic puzzles, though they're substantially less brain-breaking. Perfect Paths is another one, that lies somewhere between Spacechem and The Sequence as far as complexity.