There are many liqueurs that are used to accent the flavor of cocktails. Purchasing all of the different types would be mind boggling not to mention very expensive. Start with some of the basics such as Creme de Cacao, Creme de Menthe, Creme de Cassis, Coffee Liqueur, Amaretto and maybe some Melon Liqueur. You can always expand your selection little by little.

Absisante - Anise
Amaretto - Almond
Amer Picon - Quinine, Orange, Genitan
Anisette - Licorice
Brandies - Apricot, Apple, Blackberry, Cherry, Peach,
Chambord - Black raspberry
Chartreuse - Herbs (Yellow and Green varieties)
Cherry Herring - Cherry
Chocolate Liqueur - Chocolate
Coffee Liqueur - Coffee
Creme de Banana - Banana
Creme de Ananas - Pineapple
Creme de Cacao - Cacao (White or Dark varieties)
Creme de Cassis - Black currant
Creme de Menthe - Mint (Green or White varieties)
Creme de Vanille - Vanilla
Curacao - Orange (Orange or Blue varieties)
Fraises - Strawberry
Framboise - Raspberry (High alcohol content)
Kirsch - Black cherry
Kummel - Caraway
Maraschino - Cherry, Almond
Mandarine - Tangerine
Melon Liqueur - Melon
Pear Liqueur - Pear
Pineapple Liqueur - Pineapple
Sabra - Orange, Hint of Chocolate
Schnapps - Apple, Apricot, Peach, Cinnamon, Peppermint, 
Sloe Gin - Sloeberrys steeped in Gin
Punsch - Citrus
Vandermint - Chocolate, Mint