Stir plates are a great way to increase the potential of your yeast. By constantly stirring your yeast starter you can increase the amount of Oxygen in your solution, knock CO2 out of suspension, and keep the yeast in constant contact with the nutrients it needs to reproduce. With the power of these forces combined your yeast becomes Captain Planet! Actually what it really does is create much more yeast cells (with healthy cells walls) than your typical smack pack or dry yeast pack. Stir plates generally cost anywhere from $60 – $100 (and sometimes more). I’ve found a great thread on Homebrewtalk.com that walks you through building your own on the cheap. There is also another good one at Brewiki.org.