The Oxyd games, by Meinolf Schneider, were one of the great puzzle-game series of the early 90s. They originated in the Atari world; I played them on Mac. I played them for hours, because they were big, big and evil and full of puzzle goodness. I still have the Per.Oxyd shareware code book.
Now -- or rather, two years ago -- an open-source implementation of the game appeared: Enigma. This means you have to play it. Now. Available for Mac/Win/Linux.
(It is not, I admit, a well-chosen name. There must be dozens of puzzle games called "Enigma", not to mention Enigmo, etc. But who cares?)
Oxyd is a physics puzzler, in the Marble Madness line. You roll a black ball around by nudging your mouse. When you hit certain blocks, they open, revealing a color. Then you play Concentration. Hit two blocks of the same color, and they're done. When all the color-blocks on a level are done, the level is solved.
Simple! Of course! Not. You'll see walls and mazes. You'll fall into water and drown. You'll fall into quicksand and drown slowly (if you don't struggle out in time). You'll hit switches to open and close doors. You'll blow up bricks with dynamite. You'll find slopes, gravity, crates, one-way doors, timed doors, springs to jump walls, lasers, pipes, deathtraps, and mailboxes (evil, trust me). There are regions of high friction, low friction, and no friction. It's very tactile -- the mouse interface practically lets you feel the wood, carpet, or metal that you traverse.
In some levels, you have to steer many marbles at once. In others, you can switch back and forth between two marbles, essentially controlling two cooperating "characters". There is, in short, a hell of a lot of variety, packed into what looks like a simple tile-based game.
Enigma is a startlingly faithful reconstruction of Oxyd, considering that 640x480 was a giant-sized screen when it first appeared. The graphics have been scaled up without losing the original style. All the levels from the original Oxyd games are included; and then a big batch of new levels. And then, since it's open-source, a steady stream of user-contributed levels. The game engine is capable of emulating Sokoban, and so a set of Sokoban levels is included. Stuff like that.
I could easily spend the rest of the holiday season playing through this thing. I won't, honest -- too many other games to play. (I've barely even started Mirror's Edge!) But I could.
Note that Enigma is a fan recreation of Oxyd. I don't know how the original author feels about it; the web site doesn't say, except to thank him for the inspiration. There is a recent game which is an official descendant of the Oxyd line -- Oxyd Extra 2.0. (Free but not open-source.) I haven't looked at it.