I say some farming mod, particularly from this young chap called Xie, his mod (Xie's Farming Mod) enhances farming, giving us a more important look on the aspect of everyday life in an minecraftian farmer.
And last but not least, Metallurgy! One of the greates mods ever invented for MC, for it enlarges gameplay.
Hope you take a look at them :)
PS: Type Minecraft Metallurgy in Google and select the second link/title from Google.