Just right off the bat, if you haven't heard of Minecraft, google it. If you have, and you haven't played it, go try it out. If you've played it and haven't bought it, buy it now.
( also, just to clarify, you don't have to buy it to play on classic multiplayer servers. Just register. I'll also give a shout out to The Ultimate Maze [if it still exists] here, for having a couple of really impressive, very difficult to complete mazes. )
I was a lot more into games when I was younger (13, 14, 15), though I mostly stuck to strategy and RPG's. I don't play games very often any more, because it's not often that I feel like a game is worth shelling out $40 or $50 for (though I'll admit that since I work at Gamestop now, I'm gaining back my interest in games in general. Yay for 15% employee discounts!).
However, Minecraft is different. Markus Persson has really outdone the industry in general with this game. It doesn't matter that the textures are pixelated (though I'll note that there are a plethora of very nice add-on texture packs out there already [video example]. I'll give a shout out to mixcraft and Doku's RPG texture pack), or that every polygon is a block. In fact, that's really what makes the game unique. The whole world is made out of blocks, and you're free to do whatever you want with it. To really understand what sets this game apart, you'd have to play it. And to REALLY get an idea of how incredible it is, just take a look at some of the things the Minecraft community has built. There's some absolutely amazing builds out there.
The other thing that makes Minecraft special, is that no single playing experience will be the same. The world is generated at random for each player (you can also make as many worlds as you want), and each one has the potential to essentially cover the earth three eight times over. Though I will suggest you find some newbie tutorials on youtube if you've never played before, because the game just drops you into your newly spawned world without any explanation. I didn't, and it took me a little while to figure out how to actually mine stone (hint, cut down a tree, make some planks, make some sticks, and fashion a wooden pickaxe on your crafting table [bonus hint: 2x2 grid of planks]).
Amazingly enough, it seems like new things are discovered within the game on an almost daily basis. Things I'm sure Notch didn't intend to implement, but likely resulted as bugs. Take the water ladder for example. There's also self harvesting cactus farms, mob grinders, airlocks, and a slew of other devices which take advantage of Minecraft's physics system.
However, what I find to be the most impressive thing about this game so far, is that it already has all of these great ideas and concepts kneaded into it, while feeling as polished as an early-mid beta. Even though it's still in alpha. Notch has done a very impressive job in that respect. Sure, there's a few glitches here and there, and it could certainly perform better (honestly, a game with graphics like this should easily hit 120fps+ on a computer that can run Crysis at medium/high settings, but it's still in alpha, so that's not a big deal right now). It's because of this that I eagerly await every update, and especially it's official release, because if it's already this good at this point in its development stage, I can't imagine how impressive it'll be when it's finished.
So go and try it out. Build a massive castle or something. Or maybe a huge water slide. You're only limited by your imagination.
"and each one has the potential to essentially cover the earth three times over"
It's eight times ;)
Whoops, my bad. :P