If you are talking internal ballistics, there is only one, and it is QuickLoad. About $180 US. Other programs are for professional use and in the 4 or 5 figure $$$ range.
All the other software are just digital versions of manuals and none of them actually calculate the pressure, they just use values from tests. For internal ballistics, there is nothing free.
Also, Quickload has a very steep learning curve if you really want to understand the answers it is giving. Internal ballistics is MUCH, MUCH, more complicated than external.
There is a demo and comes with the PDF manual. The appendix to the manual is a must read if you are at all interested in internal ballistics.
BTW, there is also Load From A Disk (which I have not used) which has basic pressure data for certain cartridges/loads/powders but I am not sure how flexible the program is or if it can be changed/calibrated. I don't think it has been updated in the past 4 years either.