amok: Faerie Solitaire is free on Steam for a couple of days to celebrate Mac release.
http://store.steampowered.com/app/38600/ It has to be one of the best solitaires ever made, spent quite a lot of hours on it over the times.
It is pretty fun for a "background" game indeed, and I believe the devs are one of those indie companies that engage their userbase fairly frequently as well. Also worth noting that it's not their first giveaway of the game, and it is one of those games that you can run DRM-free after downloading it from Steam, if you so desire that option.
But they didn't release it to celebrate the Mac release, they released it because I finally got the last egg for the last achievement yesterday evening (after 77 hours spread over a year or so) and it's totally in celebration of that. :P They also released the Mac version in acknowledgement of that, naturally. *nods really hard and believably*