Cave Story is a legendary indie Metroid-style action platformer exploration game released in 2004 by Studio Pixel. It's an amazing game, and possibly the greatest game featuring happy demonic toasters ever to be released. What it did not have is ponies.
Thanks to Dirkos, that problem has been fixed. A few months back, he released Cave Derpy, a respriting of the game to feature the lovable mailmare instead of a certain soldier from the surface, creating new flavors for both Lyra and Berry Punch shortly thereafter. These works have sat in obscurity... until today.
The story and game play very much remain Cave Story, but with sprite and text modifications in place to make the game feel like it centers around your pony of choice. It's quality work, and as you can clearly see above you little pixelated Lyra is adorable as all get out. If you've never played Cave Story, this is an excellent opportunity to try it out! If you already know how wonderful Cave Story is, then why not fall in love all over again with a little puff of fresh, ponified air? I think you'll be delighted poking around and seeing all the little differences. To play it yourself, check out any of these fine links, but be sure to uncheck the custom downloader box before downloading the game!
If you've come this far and either don't feel like playing yourself or just want to see what it's all about before giving it a shot for yourself, you can also just watch me play (and generally fail/squeak in) the Derpy version: