By taxi
Taxis small white cars are plentiful and easy to use. They are almost always shared 1 passenger in the front, 3 in the back unless you ask specifically to pay more to make it private. There are a handful of places where taxis will queue for passengers airport, grande marche, etc, but most of the time you just stick out your hand towards the traffic, maybe give a weak wave, and shout for "taxi" or "taximan". The rate for a shared taxi is FIXED at 200CFA per person. Through the open window, tell the driver your destination and if he nods, or stays there, you're good to go. It is better to give a section of the city or a major landmark as the destination than giving an address. If he drives off, he wasn't going near your destination; just hail the next taxi. For longer distances the taxi driver will indicate it is double 400F by saying quatre cent French, deux courses also French or wah-haku Djerma before you get in the car. If in doubt, confirm the price beforehand, especially if you are white and take a taxi near a hotel.
The rate is 800 to 1000 CFA if you want the taxi all for yourself. You should pay more only from bus station 500 FCFA and from the airport 3000F or higher. Prices double after midnight. Often times, taxis will wait outside hotels looking for passengers, usually they will try to charge more than the official rate or are only looking for single passengers they won't stop for other passengers and will charge you accordingly
If you need to call a taxi to pre-arrange something, one to try is Taxi Bonbon at 96.96.76.46. Fun guy that likes to chat and flirt.
Addresses
Street addresses were not devised until the past decade for most West African cities. Niamey implemented one of the most efficient plans in the region between 2001-2002. The city has been divided into 44 sections named and based largely on existing neighborhoods and each was given a two letter prefix for instance, "GM" for "Grande Marche. Since the vast majority of roads lacked names, numbers were assigned to each road even if it has a name; streets running roughly parallel to the river were assigned even numbers and cross-streets assigned odd numbers. Over 100,000 street signs were installed to denote these roads at intersections. Addresses were assigned by distance from the river, alternating even-odd on opposite sides of the street. Thus the address 4735, Rue GM 12, Niamey is located in the Grande Marche district on road 12 which runs parallel to the river.
By car
There is a Hertz car rental place in Niger that rents Toyota RAV4. It is very expensive!