Originally Posted by Mike Cash 
The guy who had the Bible translated? He died in 1625.
Funny. I thought of the same fellow.
I certainly gained a new level of control when I switched to a DSLR from film so I'm not sure it's that important to have a full kit of lenses anymore. A DSLR shooter really has no need for the complete 28-35-50 trio now.
Therefore, my vote for a must-have lens is for my A1.4x tele-converter. It's just enough to shake loose the cobwebs and offers a different net field of view for lenses I've become a little bored with using.