Difficulty Locker by Matthew Smeets
Difficulty Locker allows you to lock the difficulty of multiple worlds separably in versions pre 1.8 where this functionality has been built into vanilla
Simply Paths by momnop
Simply Paths is a mod which adds many types of paths to Minecraft based on both Custom and Vanilla blocks.
Carbon Mod by jmanpenilla
Use carbon to make diamonds! Very early beta, mechanics are likely to change.