I just wanted to add a collection of high-resolution photos folks can use for things like posters. The images below are infinitely scalable, which means you can make them as big as you want without losing resolution. Click each image to download its corresponding infinitely-scalable PDF version. [Note: Just DON'T try to imbed these into a PowerPoint slideshow. These are very large. Only use it when you need to print a big something -- like a poster, like Brian said. JR]
Sources
http://www.trademarks.umd.edu/marks/inst.cfm
http://www.nsf.gov/policies/logos.jsp