OptiFine

OptiFine is a utility mod created by sp614x. Support for shaders was first seen in version 1.8.8_HD_U_F2 for Minecraft 1.8.8. Shader Packs Launcher is based on Shaders Mod code.

Downloads
All previous versions are available for download at https://www.optifine.net/downloads

They will be archived at some point in the future, but Optifine is still being updated and maintained for older versions including 1.15.2, 1.14.4, 1.13.2, 1.12.2, 1.11.2, 1.10.2, 1.9.4, and 1.8.9 (as of 2021).

Problems
OptiFine still does not work natively with Fabric. It is possible to make it work with OptiFabric, but it also needs standalone OptiFine. For shaders on Fabric, Iris can be a great tool.