All mods include the permission to share and distribute their code.
IC2 http://www.industrial-craft.net/
Galacticraft http://www.micdoodle8.com
CoFH http://teamcofh.com/
Gravestone http://gravestone.nightkosh.com/
MrCrayFish https://mrcrayfish.com/mods?id=cfm
Tinkers Construct & Mantle https://minecraft.curseforge.com/projects/tinkers-construct
Fairylights http://minecraftsix.com/fairy-lights-mod/ (CC)
InventoryTweaks http://inventory-tweaks.readthedocs.io/en/latest/ (MIT)