An Embassy is the representation of a country at the seat of government of another country. The page embassy-finder claims to show a very reliable 'list of embassies and consulates' all over the world.
Hitchhiking at an embassy
You can of course use Embassies for hitchhiking. As you need visa to enter some countries, you have to go to the embassy of the country you want a visa for and make a request for it. Other people also have to. It's your chance to talk to people waiting whether they want to go to the country of your choice and when and arrange a lift there. This seems to be good advice if you try to hitch from Morocco to Mauritania - you can ask at the Mauritanian embassy in Rabat (6, Rue Thami Lamdawar, B.P 207, Souissi/Rabat)