Jumpman 

Jumpman is a pixel-art action platformer game with 8-way gravity manipulation, wraparound levels, destructible environments, and a variety of enemies. The primary game consists of ten "paths", each consisting of a number of 2D levels. It's usually possible to preview the next level because it appears (at times confusingly!) in the background of the current level. The game tracks deaths and speed for each path, listing both the time and deaths for the run-through with the best time and best score, perfect for aspiring speedrunners.

At the start of each new level, it's impossible to know without prior experience how the gameplay features will manifest. In some, gravity is locked and old-school platforming is where it's at; in others; Jumpman is ironically unable to propel himself and must rely on world rotation. Bombs, when activated, can do anything from opening a gateway to causing every platform on the level to come crashing down. On some levels Jumpman can even bypass platforms by changing color, which makes for some truly disorienting gameplay down the line.

Sam's Protip: I find that Jumpman's repeated fall in the background of the main menu is a very effective invitation to create a world around him! There have been a number of worthwhile user level packs created as a result.