Reliable Travel Resources

IATA Travel Centre (Visas). Majority of airlines rely on IATA tools for visa requirements verification. This and government websites are two main sources of truth on visa requirements.
Wikipedia - List of Visa Requirements by Nationality. Community-driven list of visa requirements for each passport. Includes references to the source of requirements (e.g. IATA, government websites).

Wiki Voyage. Very comprehensive, travel-oriented Wiki pages for all countries, as well as their regions, cities, and attractions. Includes information on logistics (how to get there, visa requirements), travel tips, safety information, health and connectivity tips, cultural nuances. It's a must-read before travelling to any destination. It also is a better alternative to Wikitravel (which is for-profit) or a regular Wikipedia (which is not travel-oriented).

Weatherspark (e.g. for Nairobi). Great data and insights on historical weather (temperature, rainfall, sunshine, humidity, hourly temperatures). Extremely useful for picking your time to go somewhere.
Climates to Travel (e.g. for Japan). Detailed human-written guides on weather for majority of destinations, covering local nuances, best time to visit, weather events like typhoons and hurricane seasons.

World Risk Map. Detailed security risk map by International SOS with information both on country and region level.
Travel Advice (USA). Travel advice for each country issues by U.S. Department of State.
Travel Advice (Canada). Travel advice issued by Government of Canada.
Travel Advice (Ireland). Travel advice issued by Department of Foreign Affairs in Ireland.
Travel Advice (UK). Travel advice issued by United Kingdom.
Travel Advice (New Zealand). Travel advice issued by Ministry of Foreign Affairs and Trade in New Zealand.
Travel Advice (Australia). Travel advice issued by Department of Foreign Affairs and Trade in Australia.