Unless specified otherwise, everything available on this site is free to use for whatever you'd like. Go wild with modifying things, copying things, whatever. I try to remember to include an open source license with all downloads but usually forget. But that's my philosophy.
Some of my PUGs sales have different rules but they are always specified in the description.
I sometimes release new hexes as exclusives but generally within 6 months I open source the base code and put it on GitHub so that anyone can make their own.