The charm of "Minecraft" doesn't really come through so well in screenshots. Videos do a bit of a better job (especially this one of a guy accidentally burning his house down), but you probably won't truly catch the bug until you've played the game. Until now, your options to do that for free were limited to the Classic version, which only allows you to place blocks. But now, thanks to a partnership with PC Gamer, Mojang has released a demo of the full game with almost every single feature included.
So why would you bother paying for it? Well, apart from your good-natured soul which tells you to give money to people when you enjoy their work, the demo actually locks out after 90 minutes and prevents you from adding anything new to your world. Basically that gives you enough time to build a hut, forage for some minerals and get killed by an exploding monster, but building a switch-operated rollercoaster might be beyond your powers.
The demo lets you build as many of these 90 minute worlds as you want, though, so if you're looking to kill an hour or so by starting a civilization from scratch, this is a decent option.
Personally this is how I prefer to play "Minecraft." Mess around with a blank canvas, find as many nifty goodies in the nearby area and then start from scratch again to see what sort of world it drops me into next. Hopefully this generous demo doesn't discourage people from picking up the full game. If anything, I'd imagine it'll bring new folks to this cube-ridden world. Here's hoping.