Relevant links: fansite, download
By this point Cave Story isn’t exactly obscure. It’s the game that everyone points to when they talk about how great indie games can be, and they’re not wrong. I can’t offer any new enthusiasm for this title, but I’m going to post it here for the sake of completeness. Maybe there’s someone out there somewhere who never pays any attention to anything, but is for some reason one of the two people who comes to this site. It’s possible. If you are that person, you should play Cave Story.
In short: it’s a sidescrolling exploratory platformer which does the exploration part very well, the action part pretty well, and the story part… is just very well presented. Plot-wise it isn’t anything remarkable, but it puts you in an environment on a small scale, with a limited number of characters, and allows you to grow attached to the people and the place that you’re in before it starts to tear that down.
The original game was made by a single person, Daisuke Amaya, over a five year period and was released in 2004. For those of you who can read Japanese, that’s available here. For everyone else the best resource is likely cavestory.org – a fan site with loads of information in English as well as an extensive list of downloads, with the game available in many languages and for many platforms.