ReliefWeb,  is the humanitarian information alert service by UN-OCHA, launched in October 1996 and administered by the United Nations Office for the Coordination of Humanitarian Affairs (OCHA).

ReliefWeb is a source of timely, reliable and relevant humanitarian information and analysis, covering all humanitarian crises worldwide. ReliefWeb focuses on natural hazards (earthquakes, tsunamis, severe weather, volcanic eruptions, storms, floods, droughts, cyclones, insect infestation, fires, and technological hazards) and health issues. It provides detailed information, maps and updates on humanitarian crises through web service, email and report as well as new communications tools (facebook and twitter).

ReliefWeb serves a wide variety of users such as: international organizations, humanitarian aid, policy/decision makers, civil society. 

References:

ReliefWeb Website (http://reliefweb.int, accessed on March 28, 2012)

V. Grasso and A. Singh, UNEP (Dec 2011, forthcoming): Early Warning Systems:
State-of-Art Analysis and Future Directions.