The original[1] was made by the Minecraft guy. Like most of his projects, the levels are randomly generated. The levels themselves end, but there is no end to the actual game. It just keeps generating new levels.

[1]: http://www.mojang.com/notch/mario/

It's a HTML 5 version of older (Java/Flash?) game called "Infinite Mario".

edit: I'm not sure about this, but I got the idea that it randomly generates the map/levels.