DC's signature drink, the Rickey, is "an airconditioner in a glass" as Columbia Room's Derek Brown puts it, and fiendishly simple.

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 or an orange syrup)
2-3 oz of rye or bourbon
fill the glass with sparkling mineral water (pellegrino works well)

Enjoy.

Variations include the more well known gin rickey, which subs in gin instead of bourbon for a crisper taste, and to an extent, the Dark and Stormy, which subs dark rum and ginger beer.