There are no railways in Niger.
Of the 10,000 km of highways over 2000 km is paved and efforts are being made to improve some of the sections that have previously been in repair. One can travel from Ouagadougou in Burkina Faso all the way to Diffa, near Lake Chad on roads in decent to tolerable condition. The road from Niamey to "Park W" in the south is paved. The Zinder-Agadez route is being repaved after being in severe disrepair for years. The Birni Nkonni-Agadez-Arlit road is in poor shape.
The country has 27 airports/landing strips, 9 of which with paved runways.
From mid-December to March the Niger River is navigable for about 300 km, from Niamey to Gaya on the Benin border.
Taxis in Niamey run either 200 francs if the distance isn't too long, or 400 francs for going almost across the city. At the airport in Niamey though they have a monopoly and the lowest you'll get a taxi for is 3,000 francs, and that's if you haggle a lot. However, if you walk south from the airport you'll hit a main road and for 100 to 150F you can get a ride from a beat up van to the Grand Marche Main Market, luggage included.
The Nigerien government has recently set up a bus service along the major routes of the country. While taking cars is exciting and interesting, they are dangerous, extremely hot, and more expensive. Plus, they are forced to pull over after midnight due to banditry. Because these cars often only leave in the evening, it can take several days to travel a relatively short distance. The large buses are brand new Mercedes buses and they carry a soldier at night so they may drive all night long. In addition, due to their large size, they can skim over potholes that would destroy the smaller vans.
Rent A Car
There is almost no possibility to rent a car in usual sense, although in 2005 a Hertz franchise came to Niamey and rents Toyota RAV4s. Also you can rent full-size cat-cat 4x4 SUV from French: quatre-quatre with a driver/guide, but in most cases you will have to arrange with companies that organise expeditions.