I have heard about a mod that is (apparently) really fun to use. It's called Hexxit.
What is that mod about? What functionality does it add to the game?
Hexxit is not a mod, it's a modpack – a collection of mods selected with a theme in mind, preconfigured to work well together, and packaged in an easy-to-install way.
Hexxit's theme is adding fantasy CRPG elements to Minecraft. You can read more and find download instructions on the Hexxit homepage.