I have a couple of expensive games waiting to be played, including Halo: Reach, and a backlog of books to read. But what's taking up most of my free time is the open-world in-development indie game, Minecraft.
It's a graphically simple, randomly generated world (with a cool procedural algorithm that creates some amazing landscapes), in which you mine for resources, and build your way around the place using what you dig up.
You make a home for yourself - which, of course, you find you must customise and beautify - and with a workbench and a furnace, you can make tools, food and other goodies, using a simple grid pattern to create recipes for new objects.
There's a day-night cycle; in the day, simple farm animals hop about but all is relatively peaceful. But at night - or if you dig down deep, in the dark - zombies and skeletons and giant spiders appear, which you must hide from, or fight with whatever tools you've made.
It is narrative-free: which means that the best stories come from it. Search for videos on YouTube to see what I mean. It drops you in the world, and lets you discover; it gives you the tools to shape the world, and simple incentives to explore it. You soon personalise your world; you invest yourself in it, and you enact the story of your struggle to survive in it.
It will be big. Really big. It's being updated all the time, and it's hitting critical mass around now...