Recs.
Updated
SpecsUpdate
Pros
Pro Considered an Indie Classic
Created in 2005, Cave Story helped spark the indie movement in game development. This entire game was created from scratch by one developer in his free time. Everything, from plot, gameplay, assets, and music were all created by this man.
This game sparked so much praise that it received a Steam port, a 3DS port, and a Wii port. Though all of these ports have bonus content and are "remastered" versions of the game, the original game is free and for the PC.
Cons
Con Replayability
Upon completing the game, there isn't too much replay value. You could try to get different endings or get different character customizations, but the plot is, in general, the same throughout each playthrough.
However, the Steam Port "Cave Story +", the 3DS port "Cave Story 3D" and the Wii Port should all have some bonus content (though, you have to pay to get these versions of the game).