Ethiopian is one of the few airlines offering a direct flight from the US to Africa. They fly from Washington DC (IAD) direct to Addis Ababa. The airline recently added the Boeing 787 Dreamliner to its fleet, the first of its kind in Africa. The Boeing 787 will be flying the Dulles - Addis route as of September 20, 2012.
Ethiopian Airlines has a very extensive network within Africa along with reasonable rates. They tend to undercut most airlines by at least $300 on routes from the US to destinations like Lusaka (Zambia), Dar es Salaam (Tanzania), and Mahe (Seychelles) to name a few (check all special fares). They serve 69 international destinations on four continents. If you're looking to fly anywhere in East Africa, or wish to combine East with Southern Africa, they are a great option. Ethiopian Airlines is also expanding their service within West Africa, providing a nice link for those who need to be on both coasts.
Extra Tip: If you're searching for airline deals to Africa, check out Turkish Airlines. They have really expanded their service to the continent this past year. Spending a few days in Istanbul as well as excellent fares make this option very worthwhile. They have flights to 17 destinations in Africa that includes the usual hubs of Nairobi, Cairo and Johannesburg, but also Kigali and Mogadishu. Layovers in Istanbul are reasonable if flying straight through from the US (which you can do from Chicago, Los Angeles and Washington DC). Click here for deals on Turkish airlines.