This is an archive of all the latest OptiFine versions for every latest major Minecraft version.
The last two characters (e.g. “A1”) are OptiFine revisions specific to a single Minecraft version. Only the latest revision will be preserved here.
“HD_U” stands for “Ultra”, which is a combination of the former separate feature sets “Smooth”, “Multi-Core”, and “Anti-Alias”.

Earlier versions came in different varieties:
“HD” denotes the basic variety excluding any of the following features.
“HD_S” includes the “Smooth” feature, which is a framerate stabilizer.
“HD_MT” supports multi-core rendering and always includes “Smooth”.
“HD_AA” stands for “Anti-Alias” and also adds anisotropic filtering. It excludes the other features but is included in the “Ultra” variety.