Basic Economics Lesson...

While standing on one foot :)  Via reader and friend Simon M.:

Math-for-Programming Primers...

Here's a nice collection of very terse math primers on a bunch of different math topics that are frequently used in programming...

Algorithmic Trading...

Here's a good primer on the basics of algorithmic trading, something that practically every programmer involved in electronic trading considers doing at a personal level.  It's harder than most people think it is!

There's a plug in here for Interactive Brokers, the company that acquired FutureTrade, whom I used to work for...