Markus Persson's Minecraft is a block-based sandbox building game originally inspired by Infiniminer, but one that requires no installation of any software to play. A Java-enabled browser is all that is needed, and though the current alpha version does not have multiplayer features yet the developer has promised that new single and multiplayer game modes will be added very soon.

You can add or remove blocks using the left mouse button. Right-click to switch between removing or adding blocks, and use the numeral keys 1, 2, 3, 4 and 6 to change block type. Creative players can build anything from houses, mazes, towers and even castles with their own moats and bridges with just a little time spent on construction.

Minecraft development blog