I just heard a great podcast of Arthur Benjamin doing his mental math tricks. The second half of the podcast is him explaining how he does it, which I think is much more interesting than the tricks themselves to those of who are math-inclined.
Something I found really interesting is that he does math by hearing or words, rather than visually. I'm visual, which is why I can do math in my head about as fast in Spanish (which I learned as an adult) as I can in English (my first language). Benjamin's room full of geeks appears to have been overwhelmingly visual as well, but he says that most rapid mental math calculators (humans) are auditory.