Penny Arcade strip filled you with an interest/mortal fear of Markus Persson's indie sandbox title Minecraft, you'll be pleased to learn that the game is free to check out today (and possibly even a bit longer). On the game's official site, Persson reported, "Minecraft got way too popular for this humble server," leading him to temporarily shut down a few of the game's online services. Luckily for us spendthrifts, Persson decided to institute "a 'free-to-play weekend', or however long this will take to fix."
You'll have to purchase the game and register a new account once things get back up and running, but this seems like a pretty great chance to see what the game's all about without paying for it up front. That is, if you couldn't suss out from the title that it mostly focuses on mining, and then crafting.