Spelunky is a fairly traditional sort of game in that it resembles a platformer. It's a 2D sprite-based game where you explore a vast network of underground caves, fighting off monsters and collecting treasure. What's unusual about it is that the cave is generated afresh each time so it's always a journey into the unknown.
It's a cute and fun game, excellent for a short gaming session with enough appeal to make you want to come back. It feels a bit like Rogue or Nethack in that you're digging though procedurally-generated caves and gathering up goodies, but there is actually an end to it. At the bottom of every level is a doorway through which you can escape. I haven't actually managed that yet. The game cleverly uses your own greed against you. You'll linger in an area you shouldn't in order to snag a bit more treasure and end up paying the price.
It's free, it's tiny (under 10MB) and you can even download the source code (although they are Game Maker files, so you'll need that to properly explore them). There's an XBox version imminent with much improved graphics.
HyperRogue III has a terrible title and it's another game that is a bit like Rogue - you're wandering through a dungeon, collecting treasure and killing creatures. The land is split into different areas and you move through them encountering different baddies in each one and finding unique treasures. It's effectively turn-based and played entirely from the mouse.
The thing that makes the game really stand out is that it plays out in non-Euclidean space - the landscape rolls under you like it's being pulled over the surface of a sphere. Download it to see it, rather than have me try to describe it.
It's free, it's tiny (500K!) and it comes with the full C++ source code.