Palmer drought severity index is one of the most popular moisture/drought indices around. There is a package in R called a SPEI that allows calculation of two other popular drought indices (Standardised precipitation index and Standardised precipitation and evapotranspiration Index ). I think there should be some package to calculate other moisture indices and particularly PDSI but i have been unable to find one myself.
It looks like we can thank Christian Zang for finally solving the problem: https://github.com/cszang/pdsi
Edit from 2018: This answer was just accepted so I have decided to fork the above-linked repo to my github to guard against future link rot. It will stay there indefinitely.