In a small saucepan, bring sugar and water to a boil; stir to dissolve. Let syrup cool completely. Combine lime juice, light rum, syrup, plus more to taste, and ice in a cocktail shaker; shake well. Strain into a chilled cocktail glass. Garnish with half a key lime.
Really easy, and surprisingly good. I will probably add seltzer next time to make it a little less potent so I can enjoy a few more.