Mike Cohn, author of User Stories Applied (which has been getting great reviews on Amazon, by the way), has an excellent presentation on Selecting an Agile Process: Comparing the Leading Alternatives (PDF, 837k). It is the slide deck from a half-day tutorial he does that includes overviews of the main agile processes: Extreme Programming, Scrum, Feature-Driven Development, Crystal, and DSDM. Mike does an excellent job presenting the essence of each agile process in just a few slides, with lots of pictures (I'm a picture kind of guy). Highly recommended!