Yes tclark is right. BUT you have to buy Film Magic Pro in order to get the tutorial. It comes free with it. I bought FMP and the Sin City tut is actually really cool! Another great buy from Andrew Kramer.
I know you can do this using almost any particle emmiter. Try using CC Particle World, CC Particle Ststems 2,or Particle Playground which are all included with AE and try playing with gravity settings and what not.