Infocom
The link given to the
Zork Anthology on GOG gets you half a dozen of Infocom's text adventures on OS X - many of the best (and Infocom was the king of text adventure games).
If you've got an iOS device around, then the
Lost Treasures of Infocom (also see the
publisher's website) will get you Zork I for free, and a $10 in-app purchase will unlock, if I'm counting correctly, 26 more of Infocom's text adventure games - I think that's everything they made except for
Hitchhiker's Guide to the Galaxy which is, indeed, sadly trapped by rights issues, and it includes
Leather Goddesses of Phobos, which used to be hard to find.
In general, if you search for "Lost Treasures of Infocom" at Amazon (or similar), you'll probably find various versions floating around (hint, track down other names for the same collection in the "see also" or "folks also bought" sections on Amazon), as Infocom's games have been republished in various collections over the years, which have gone out of print, only to be republished again later. If you're handy with poking at files, the needed game files can be extracted from any of these versions (e.g. an old DOS version), and fed into one of the "Z-machine interpreters", like
Frotz, to play on almost any platform (seriously, I had these running on my Palm Pilot, back in the day).
What you're looking for, in general, is called
Interactive fiction these days (back when we were playing the original Zork, there was just
Zork, and
Adventure, and no categories - plus, we rode around on dinosaurs). If you google for "Interactive fiction", you'll find the
Interactive Fiction Archive and the
Interactive Fiction Database - which could keep you reading/exploring for weeks.