Browsing through our current tags, I see we have open-source (even with a tag wiki already), and floss (which I just learned stands for "free/libre open-source software"). The latter IMHO is a rather uncommon (or at least less known; before yesterday, I only knew FOSS for "free and open-source") term for the same, as open-source generally is "libre" (you have the liberty to get the code and poke with it), and mostly free (except for donations).
I know there's a related discussion ongoing with Tags for free software and free software?. But AFAIR this "synonym" was something already agreed on (i.e. it was agreed both terms are used synonymously – not on the fact to create a synonym ;)