Minecraft wiki entry for Demo, associated text: A Minecraft demo mode of the current game version for users who have not purchased the game yet and would like to try it out first. They can play on a world for up to 100 minutes.
The PC Gamer demo (outdated, but still available for free) - a limited version of Beta 1.3 that can be played on a world for up to 100 minutes.
Classic (outdated) - the first development version of Minecraft available in both Singleplayer and Multiplayer.