There are plenty of drink apps in the App Store, but I particularly like Drinks because it allows you to search for drink recipes by ingredient. Say a peek into your liquor cabinet reveals that you only have Brandy and Rum in stock, you can tap the "ingredients" tab, and Drinks will give you a list of cocktails that can be made with those ingredients (Boston Egg Nog!).
Drinks includes over 4,500 high quality cocktail recipes, includes both popular and lesser-known drinks. It even includes the type of glass, garnish and lets you to add your own recipes or tweak theirs to your liking. You can even add notes about how your Dad like his Tom Collins.