If you want to fully experience all that Minecraft has to offer, you will need to purchase the game. As I explained above, there is a Demo World that can be played for free, but it only lasts for 5 Minecraft days.
However, Minecraft did re-release the classic Minecraft for free as a browser-based game. The game is the original 2009 version with just 32 types of blocks. Slow play only has Creative Mode and all the original bugs.