FreeCiv obviously!
And any Demo's! Between all the freebie demo's I hardly need to buy games anyway... I've been playing the UT2004 demo, the Call of Duty demo, the Space Colony demo so much lately that I hardly have any time left for the full versions I own ...
And, well, uhm, didn't we have a thread just like this one some time ago?
Primate Plunge is excellent, but too easy. Very time consuming and hella-addictive, though, I warn you. Don't have a link, but I got it from MacUpdate I think.
I also enjoy playing Encompass, Aquatomic, Snood, and OF COURSE, Glider Pro, which is now officially abandonware, and can be downloaded from the author's website - takes me back to System 7, that game does.
Yes! It's a great game....very action-packed as you went up in levels. I'm sure you can find it in some store as part of a classic game pack. You might also want to try and find some clones of Tempest that might be free to download, as I doubt that the original would be made freely available.
I don't mean to break any rules, because I don't know the legality of the thing, but "Shufflepuck Café" runs perfectly happily under Classic, and is quite readily available on the net. I figure that since Brøderbund closed, it's not particularly fussed about things.