According to Wikipedia, the half smoke is a half beef, half pork smoked sausage, bigger in diameter and spicier than a hot dog. The etymology of the name is uncertain but the most common explanation is its half-and-half meet composition. It's served on a bun, often with spicy mustard and sauteed onions. You can also get it with chili.
I simmer my half smokes in a good beer for about 15 minutes (like cooking with wine, don't cook with a beer you wouldn't drink), then finish them off on the grill. I cook several slices of bacon, then saute the onions in the same pan. Spread spicy mustard on a toasted potato roll, add the half smoke, then top with the onions and crumbled bacon. Enjoy.
Half smokes are packed in natural casing so they give you a nice pop when you bite into them. The next time you visit Washington be sure to try a half smoke.