Satin is a simple library allowing modders to add interesting graphical effects to their mods. Its main feature is to significantly simplify the use of Minecraft's data-driven shader system (that is, the one used by super secret settings, mob vision, and spectral arrows).
A full feature list with installation instructions is available on the github repository : https://github.com/Ladysnake/Satin/
You can find documentation for the shader format on the wiki : https://github.com/Ladysnake/Satin/wiki/Data-driven-shaders
FAQ
Can it replace Optifine ?
No. Optifine shaders are significantly more complex than what this library allows. For a work-in-progress alternative, see the Canvas Renderer project.
Can it bring back Super Secret Settings ?
Satin is just a library, so it does not add any content by itself. You may however be interested in Secrets No More, a mod using Satin to do exactly that !
Does this library have a Forge version ?
No, and I do not want to port it myself. I will however support anyone who does so. Also note that the documentation for the shader format stays applicable regardless of the mod loader being used.
External resources
Project members

Ladysnake
Pyrofab
Main Developer