Open Software is not bad, but IMHO bringing the thesaurus, you'll miss both the "simple" and the "freedom" part.

BTW, I see all english terms on this subject being a bit misleading, that way or the other.

Let me point out that as a non native speaker, I don't have a clue how to solve it. I feel confortable using english terms at work (spanish terms regarding to computing in general just plain suck), but this case is just the exception of that rule. This one the spanish term is way better than its english counterparts.