I like goober in my coffee. Anything from a sweet liquore to some sort of bottled Torani or Da Vinci coffee flavorings. Then I need some sort of cow product to lighten it. I don't drink coffee every day but when I do drink it, I live to have it burger king style.
1 of my guilty pleasures when I'm in Europe is go have a cappuccino corretto with a cornetto for breakfast. I find for me that I prefer an espresso brewed coffee over any other style of brewed coffee. It has a great aroma and taste to go along with the overall creaminess of the coffee. I also like the kick that I get from it.
Iced coffee is a nice summer time beverage enjoyed any time of the day. Some of the other things I've put in my coffee include:
Sambuca, Anisette
Frangelico
Amaretto
Bailey's
Kahlua, Tia Maria, etc.
Galliano
Strega
Irish whiskey already mentioned here:
Buena Vista
Brandy
Lemon zest