I have been using Photo Buddy to calculate the shutter speed while using ND filters. It does the job, but it's a little clunky as ND calculations are just one of many features of the app.
Today I saw a recommendation from +Matt Kloskowski
for NDTimer, so I paid the $0.99 to check it out. It's perfect! So clean and simple! Great app!
Check NDTimer out if you're into long exposure photography.