MouseDelayFix fixes the mouse delay bug introduced in Minecraft 1.8 (MC-67665), which causes your actual block and entity interaction position to be one tick behind your crosshair.
Gif comparing the regular and fixed block selection at 5% game speed by Hink:
Video explanation by Semx11:
Simply place the jar file in your forge mods folder as you would with any other forge mod. The jar should work with all Minecraft versions from 1.8 onwards. Delete the jar file to uninstall the mod.
MouseDelayFix will crash Minecraft 1.7.x if it is present in your mods folder. This is unavoidable due to changes in forge 1.8, sorry!