Riverkeepers, watershed associations, and other groups that track water quality should be interested in IMRivers. This interactive mapping tool is very impressive! Unfortunately, there's no information on their website about costs, so I'm waiting to hear back from Dr. Wansoo Im, the tool's creator.
One of the latest groups to start using the tool is the San Diego River Park Foundation.
Some other organizations have started using IMRivers for other purposes: the Lower Columbia River Estuary Partnership's map lets you view images along the river's shoreline; the Delaware & Raritan Canal Community created their own interactive photo album, and Chatham High School is using it to map its student-led water quality testing project.