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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.
Pro Multiple Endings
For being a free game, the amount of content is astounding. There are a total of three endings, and there are a lot of extras that you can get as well (as well as a hidden area). These endings are determined by your choices throughout the game.
Pro Gameplay and Content
Extremely fluid controls and interesting level mechanics makes this game a joy to play. Furthermore, there is about 4-5 hours of content to this free game.
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).