Just found Kegulator via http://www.techcrunch.com/2007/03/06/the-much-needed-beer-calculator/

Solves the problem of figuring out how much beer to buy before a party. Neat idea, although my experience with keggers is that you’re not really sure how many people are going to show up in the first place. Mashup this functionality with the RSVPs from an Evite, Facebook event invitation, or other service, and we’ll be really talking.