I just stumbled across a great site called DonorsChoose.org and thought it was worth mention. In a nutshell, teachers across the country can address needs they have for various projects, and if any of those projects strike a chord with you, you can help that teacher out with a donation or gift certificate.
Projects vary widely from needing $40 or $50 for a few books in the classrom to tens of thousands of dollars for science equipment. What is especially interesting is that you can choose whether you want to target your donation to a specific region, to a specific school, or to a specific project that you believe in.
Here are a couple links to projects that teachers in the Denver metro area need help with: