I love the Mr. Malty calculator. But after reading "Yeast" I got very curious about how much yeast my starters were actually producing, particularly when I step them up. I couldn't really find anything available on the Web, so I made my own application, and decided to make it available to the home brewing community.
I built it around the data from CW and JZ's book "Yeast", and it correlates very well with Mr. Malty.
http://www.yeastcalc.com/