Here are some links I found useful:

Guideline.gov's PDA link is http://www.guidelines.gov/resources/pda.aspx

Comparison of Lexi, Epoc, and Micromedex @ http://www.hshsl.umaryland.edu/resources/pdainfo/rx.html

Ohio COP: http://www.pharmacy.ohio-state.edu/services/comp/pda.cfm

and of course, my favorite: http://www.davespda.com/