The MDD is the last Macintosh capable of natively running Mac OS 9. (Although, only just. It came with a special build that was just for the MDD.)
The fans in it are also astoundingly loud, and was often given the nickname of the "wind tunnel" Mac, because of the holes in the front.
Allegedly, there was yet another model after this, which wasn't advertised publicly, sold exclusively to creative professionals. (Mostly because of Quark XPress, which took years and years to transition to OS X, which also opened a large window of opportunity for InDesign to become the new standard.) But I have no reliable source for this.
Once the G5 came out (without support for booting OS 9 bare-metal) they continued selling special config of the MDD for another year.