Kirs are popular drinks made from dry white wine flavored with cassis. The drink, incidentally, is named for Canon Felix Kir, one-time mayor of Dijon, France (the center of cassis production).
warrenr explains: cassis and creme de cassis are names for the liqueur made from blackcurrants steeped in neutral spirits, with lots of sugar added. Two favorites available in the US are Cartron Double Creme and Lucien Jacob. Also good are Trenel and Theuriet. L’Heritier-Guyot is the most widely available brand, and it’s marginally acceptable. Most domestic stuff is pretty poor, though Warwick Winery has recently started production of a pretty good cassis.
Maxwell also recommends G. E. Massenez de Dijon, though you may need to go to Dijon to find it.
Bear in mind that freshness is important with cassis, so buy from a store with high turnover.
Board Links: What would the premium brand for Creme de Cassis be?