I'm bummed Shimano seems to have discontinued the Mega 7 freewheel that sheldon raved about here:
http://sheldonbrown.com/mega7/
I was so taken with his description that I have a new 11-28 in my parts bin waiting for the right wheels.
I checked ebay with a search on "Shimano freewheel 7" and there are plenty of 13-28 blocks out there.
Also several 12-21 dura ace described as "like new," and you know why.