The Lethrys Upscale is a built-from-scratch, 2x-res upscale of Cave Story, intended for community use.
More info can be found at the GitHub page, or on the Tribute Site Forums thread.
Freeware builds can be found on the Doukutsu Club page.
Also, this upscale is being used in the Cave Story ~ Encore project. Check that out if you'd like.
Why did I do this? Here's why: