Gem Cave Adventure
Imagine an entire game built around the opening scenes of Raiders of the Lost Ark and you'd have Rastatronics' Gem Cave Adventure, a bare-bones platformer that puts you in the role of a jewel seeking adventurer, exploring your way through 50 caves, dodging obstacles using the [arrow] keys, swinging past treacherous falls by pressing [spacebar] to deploy your whip. and activating ancient switches using the [S] key. The only problem is that, once you've taken your treasure, ancient traps activate and you'll have to make it past them all if you want to cash in your find.
There are also lots of side quests and extra gold chests, that, along with your gems, add up to money you can use to construct a town above ground from the caves where you do your adventuring, but this feels a bit tacked on, and most players will only be interested in the straight platforming challenge of it, which is a nice, smooth curve of difficulty that will leave you impressed, but not frustrated, even if you can't beat the last levels, which are quite difficult. Gem Cave Adventure doesn't do anything new, but what it does do, it does perfectly.