I agree with a lot of names already thrown up here, especially Greg Oden.
The question is sort of interesting because of the relative nature of "underrated". At the top end of the game, I agree Dirk is underrated relative to other top players (here are two others: Carmelo Anthony, Antawn Jamison).
If the question is more of a wing man type player who is underrated (no I don't mean wing as in the position on the court, I mean as in... wingman), I agree with the idea of Scola, who is underrated relative to Yao/Artest/McGrady... (here are others: David West, Jason Terry; Jeff Green may soon be a candidate here).
Looking more at role player types: I agree with Pryzbilla; my personal favorite: Ryan Gomes.