Minecarts are a vanilla item that is used to ride down Tracks. They are also used to craft a few different types of carts from the RailCraft mod, and is also used in crafting the Tunnel Bore, a machine used for making perfect 3x3 tunnels.
With 5 Iron:
With 5 Bronze:
A Minecart linked to a Furnace and Chest Cart.
With 5 Steel:
Tunnel Bore
Tank Cart
TNT Cart
Work Cart
Chest Cart
MFSU Cart
BatBox Cart
MFE Cart
Anchor Cart
Furnace Cart