Any player can download and install an unblocked "Minecraft" demo directly from Minecraft.net. This version is freely available to the public at all times. To download it, visit the website and register for a free Mojang account and then download the game.
After you download the "Minecraft" software from the website, install it. When the installation completes, start the game, and enter your account information. Once logged in, start the demo by clicking on the large Play Demo button at the bottom of the window. Each play session is limited to a total of 100 minutes of play, although you can reset the map and play again as many times as desired.