Wikipedia has a good writeup of the Dark n Stormy, but it is a classic Caribbean/seafaring drink - dark rum, a bit of lime, and ginger beer (DG brand works best).

In a highball with ice, add in:
The juice of a key lime (and both shells, or the juice of 1/2 of a normal lime and a dash of simple syrup)
2-3 oz dark rum (Myers or Flor de Cana)
If your ginger beer is not sharp enough, consider adding some ginger syrup.
Fill with Ginger Beer to taste.

A slight dash of vanilla can also amp it up.