TouchpadPal is a free program that prevents accidental touchpad activity when you're typing. The program requires no configuration -- just install it, and it goes to work right away.
The program doesn't actally turn off your touchpad, so you can still use it as a mouse substitute. But when it senses you're typing, it intercepts touchpad commands that it thinks are spurious.
I actually wish TouchpadPal was more aggressive. It still lets some unwanted touchpad commands through, so even if you're using it, you might find your cursor move occasionally. But it's a good start. For other alternatives, check out my older post on the subject, Disable Your Laptop's Touchpad When Typing.[via Digital Inspirations]