Never understood this tbh. I would never consider risking my life on something I was using becuase it was free. Seems like a fast way to get yourself in the hurt locker.
Anyway
have a look here
http://directory.google.com/Top/Shop.../Scuba_Diving/
I think GAP is free, as is ZPlan. Abyss, V-Planner and a few others are free to try out, but V-planner is not a lot of money for something that a lot of people trust.