Little known fact - Jumpman was Mario's original title! This indie game is just as exciting as the early platform games, and its blocky graphics makes it decidedly retro.
You control a man... who can jump! Each room has an exit you have to reach, and you can see the rooms ahead of you, as they are all laid our behind your current location. Fin an exit and you zoom into the next room. There are the usual enemies and life-threatening objects, all which kill you on touch.
There are also some cool innovations, like allowing you to rotate levels and the Superball which kills everything it touches, so you can use it, carefully, to clear enemies from a room. The controls are simple, and configurable. The physics are pretty light - Jumpman skids and slips around, which makes him pretty tricky to maneuver. That takes a bit of getting used to, but you'll soon get the hang of it.
Jumpman doesn't need installation, there's a level editor and a neat 'playground', which lets you create rooms and play on the fly. Because it's so minimal in terms of graphics and sound, it may not be to everyone's tastes, but overall it's such a well designed and playable little game it's hard not to like.
With its basic yet addictive gameplay and lots of features, Jumpman is one of the best indie platform games around today.