I too was surprised to see that apparently, not one Slytherin remained... except that I think Slughorn helped in the defense of Hogwarts, didn't he? He's the only one that comes to mind... isn't there a sentence about him leading a group of the defenders? And I guess one could argue that Malfoy contributed, although unintentionally. And Crabbe or Goyle conjured the fiendfyre that destroyed the diadem. So I guess, except for Slughorn, the Slytherins helped only by accident.
Don't forget the guy that basically put himself through hell his entire life to work against Dumbledore: Snape
Oops, make that Voldemort, not Dumbledore