I had a blast participating in the Pine Street Inn Knit-A-Thon last year. (Note that the website has not yet been updated for the 2010 knit-a-thon, but I was assured at the event last November that there would be one this fall.) In 2009, I was able to complete one entire afghan for the cause, in addition to some extra squares.
This year my goal is to make 2 whole afghans, which is 70 squares! I think the limiting factor will be the amount of yarn that I have available. The current tally is 6... 64 remaining!
Pine Street Inn website for more information on the knit-a-thon!