The following countries can obtain a visa on arrival for EUR 50.
Austria, Belgium, Benin, Burkina Faso, Canada, Cap-Verde, Central African Republic, Congo, Cte d'Ivoire, Denmark, Finland, France, Gambia, Germany, Ghana, Great Britain, Greece, Guinea-Bissau, Guinea Conakry, Ireland, Israel, Italy, Japan, Liberia, Luxembourg, Mali, Mauritania, Mauritius, Morocco, Netherlands, Nigeria, Portugal, Togo, Tunisia, Sierra Leone, Spain, South-Africa, Sweden, Taiwan and U.S.A.
Even if you don't need a visa we recommend you obtain a visa before arriving as visa rules can change without warning.
Everyone else will need to obtain an E visa before arriving; http://www.snedai.sn/paiement/
All Visas cost EUR 50
Foreign embassies in Senegal