Trine is a terrific puzzle/adventure game that's akin to Blizzard's old The Lost Vikings title, if you remember that one. It features three different characters (a thief, a knight and a wizard) all controlled by one player, each with different abilities that are used to navigate various physics-based puzzles and traps. It was a solid indie title, and did well enough to inspire a sequel in Trine 2.
Trine 2 took the same concept even further, featuring updated graphics and the addition of a co-op mode, in which three different players could take over the three different roles and upgrade them as you play along together. Just like the first title, Trine 2 offers physics-based puzzles, and a really gorgeous mix of action and intellectual gameplay. This is a true indie gem -- you can tell there's a lot of love put into this one, and while it falls down in parts, the overall package shines with polish and ingenuity.
You can get Trine 2 on Steam or from the company's main website, but it's currently on sale on the Mac App Store for just US$3.99, 75 percent off of the usual cost. Note that you will need to have a fairly new Mac (basically 2010 on) to play it, as the game can get graphically intensive at times. But if you do have a newer graphics card, this game is a no-brainer for just $4, and the ability to play online co-op with friends is icing on the cake. There's also an expansion pack called Goblin Menace that's out on Steam right now -- presumably that's headed to the Mac App Store version soon as well.