Ethiopian Airlines is one of Africa's largest and best airlines, with an extensive network of flights throughout Africa, as well as to Djibouti and Yemen. Departure tax is
You can get to Ethiopia from Djibouti by road, but it's pretty exhausting - a bus-train combination tends to work better. Buses and trucks run between the border town of Moyale and Nairobi, but the trip can be slow.
Ethiopian Airlines has a strong internal network and its prices are reasonable. There's also a good network of cheap, slow buses along all major roads. In the bigger towns, you need to buy your ticket a day beforehand or you might end up paying double to the touts who occasionally buy up all available seats. The only train line runs between Addis Ababa and Dire Dawa, but it's a long and slow haul.