If I use PokerStars software in the US (not in NJ or Penn) only to play a private, free game (no money), is that legal?
pokerstars.net is something that I use for friendly play-money tournaments within the states (CA to be exact). It is 100% legal to play any kind of poker in the US given that there is no real money involved whatsoever.
There is no ban on the software, you can even download and play poker for play money on pokerstars NJ/NV. The illegality comes from using real money, which is impossible to do without violating TOS on stars anyways (given you are in the US).
this is a site I like to refer to for laws regarding online poker on a state-by-state basis (specifically the section entitled "state-specific gambling information"). Of course this is not an official government site, if you would like official government information about the legality of a specific game you are thinking of playing/running, I would suggest giving your state gaming commission a call and asking specifically if what you are planning is legal or not. It requires a bit of hoop jumping to reach someone able to answer your questions (at least for California) but if you are really interested in finding official government information then that is the best way to go.