I’m wary of free services but I use DrFirst which I think is the same as iPrescribe. People have had horrible customer service with them but I didn’t experience it and it was pretty easy to get set up. It’s not free for the PDMP in my state though and I believe there was a one time fee for a few things to get set up.
I’ve heard good things from others about MD Toolbox, OnCallData, and allscripts but these are paid services.