Lonely Planet online is a website based on the Lonely Planet travel books. This link will bring you directly to the travel information page about Niger. Here you will find a good deal of travel information to start your exploration of Niger. The information is organized in the same manner as the books (which you can also order online here). The background info is organized by topic: Weather, Health & Safety, Government, People & Society, Culture & History, Time & Place, Crossing Borders, Money, Events and Transport.
Nice guide by Wikitravel about Niger. It's an user generated travel guide supervised by administrators. On this page you will find the following travel tips about Niger: regions, cities, entry requirements, getting out, languages, eating, drinking and sleeping, working in Niger, health and safety, respect and contact. Though wikitravel updates the information frequently, we suggest you always check the travel advice from your own government first, before leaving your country.
WTG provides detailed travel information and articles about Niger. In the left menu you will find guides sorted by theme and with the menu on the right side provides the table of contents for Niger: overview, passport and visa, money, health, accommodation, going out, getting around, climate, map and images, travel advice and more. You can also create a miniguide that you can print.
World66 is a nice user generated travel guide made by travelers. Every part of the travel guide can be edited directly by clicking the  button. The guide about Niger contains information about the following travel topics: general information, map, history, practical info, economy, health, people, books and a photo gallery.