Ever since minecraft 1.9, the luck effect has been hidden in Minecraft's code, a latent, unfinished mechanic that presently is unobtainable and only works with fishing. Well, not anymore! This mod actually makes the luck effect useful, and also adds recipes to brew its potions.
How it works is simple; when a block is mined or mob is slain, the mod adds the player's luck attribute the current level of fortune/looting. As such, even unenchanted tools/weapons can yield extra drops as long as luck is active. This also works for Bad Luck, though in reverse, as it reduces your fortune/looting level. The effect stacks on top of enchanted tools and weapons, so if you drink a luck potion, your fortune 3 pick now is effectively fortune 4.
Here is a video showcasing the mod in action:
Additionally, the mod adds recipes for potions of good luck and potions of bad luck. The potion of good luck is brewed with a four leaf clover, obtainable by breaking grass with a fortune level of at least 1. Each level of fortune grants an extra 1% chance to get a clover. The potion of bad luck is made by inverting the potion of good luck with a fermented spider eye. Both potions last for 1 minute, but the effect itself is vanilla, so the vanilla luck potion will work with the mod too. Thus, if you have a mod that lets you get vanilla luck potions (such as Charm for example), you can use those for a longer lasting version of the potion.
Modpack policy: Go ahead, just be sure to link back here!