Let's not forget Deep Impact. Flew past two asteroids, and even threw something into the first one while taking pictures (with an incredible autonavigation system). Pretty awesome if you ask me.
That was pretty novel, but I still think Dawn is one of the most interesting still because of the ion thruster and re-orbit of ceres. The neat thing about DI (now called Epoxi and run by UMD, btw) is that it's now become a sort of flying testbed for all sorts of experimental flight and communication software. See "DINET"