I've talked briefly about Mental Math tricks before, but I've never really had a good source for a large variety of mental math feats.

I'm here to tell you that thanks to some like-minded fellow geeks, that problem has been solved.

First, there's BEATCalc at the Math Forum. BEATCalc's main incarnation is as a newsletter sent out regularly with a trick that will let you impress your friends by doing math quicker than they can do it with a calculator! At this writing, they list 518 math tricks in all. If you want to get started with some of the impressive ones right away, my personal favorites are "Adding a series of ten numbers" and "Adding a series of doubles".

Another great read on mental math is "The Psychology of Rapid Mental Calculators". While there aren't as many routines here, the caliber of the featured effects is much higher. The dice that are mentioned in the Magic Dice Trick routine, it should be noted, are available commercially and inexpensively.

There is also a good selection of some impressive mental math feats over at Mentat Wiki (which I also recommend for its memory feats, of course), including alternative many alternative techniques taught in the above Psychology article.

However, for a comprehensive course in amazing mental math is the Vedic Mathematics course. No, you don't have to change your religion, and it's not "new math". It's just the old math, taught with some amazing shortcuts. In Vedic mathematics, there are several "sutras", or general principles, which are taught in general, and then how to apply them in specific cases. If you actually take the time to study and practice each step, you'll be amazed how quickly you'll pick up the principles.

After going only a short way throught the Vedic Mathematics course, I couldn't help but shake my head and wonder why we aren't teaching our children things like this and memory techniques in our schools. On the other hand, the longer these tecniques are considered unusual, the longer you and I can amaze others with them!