I came back here to see if anyone had posted the DW Kitbuilder (which they have) but this is insane. It not only lets you hit the drums as Bonzo mentioned, but it also has a sequencer to play patterns back in loops. And each drumshell is sampled, so you can hear the differences between models and sizes. Some features don't seem to work on Opera (and probably Firefox) so you may have to use the dreaded Internet Exploder™ just for this.
Quite possibly the coolest drum-tool I have ever seen online. Great find!