Amy Jo Kim gave a talk at eTech called "Putting the Fun in Functional". Bruce Stewart wrote up notes on the talk.
Great tips stolen from game design. Amy convincingly argues that you can get users to do things that otherwise might be considered work by making the tasks fun, by making effort feel like play.
It's a good contrast and counterargument to one of my common themes, that any attempt to get users to do work are doomed to failure.