I hadn't had a black and tan for a while, so the other day I ordered a couple. If you are unfamiliar, it's a pint of two beers, Guinness and Bass, with the darker Guiness layer floating on top of the lighter-colored Bass. Here's a picture.
Bass has never been one of my favorites, but it was worth it just for the visual pleasure. There are other mixes, too. A snakebite is stout layered on top of cider, and a half-and-half replaces the lower layer with lager. They're a nice change of pace and they draw attention from beer neophytes if you feel the need to look pseudo-sophisticated.
For the most part, however, I'm gonna keep my beers in separate glasses.