One of the big natural disasters we've been waiting to happen here is a major earthquake. Ushahidi could help with resource sharing post-quake. However, this would require some form of communication if cell towers were downed.
Info on water and food would be the most useful, as well as transportation or emergency/high occupancy building collapses that needed immediate attention.
Additionally, info shared on which areas to stay away from in the incidence of flooding or riots would be useful- it could act as a service for stopping and isolating mobs.