Cornell Lab of Ornithology is located at Sapsucker Woods in Ithaca and is a world-renowned institution. Its Macaulay Library is a scientific archive of audio, video and photographs of the natural world.
The nature section of New York state's Department of Environmental Conservation website provides information on animal, plants and aquatic life, as well as forests. Links on both of the pages lead to identification guides and other useful tools.
The Biodiversity Heritage Library (BHL) is a global online resource that can provide information on the flora and fauna of Cayuga Lake or surrounding counties. Use the search box provided on the site.
The Flora of the Cayuga Lake Basin is just one of the many documents in the BHL. It was produced by the Cornell Agricultural Experiment Station in 1926 and can be searched for a specific location.
Global Plants is a database of digitized plant specimens. Not specific to Ithaca, but a good source for images.
Want to know what prehistoric Ithaca might have looked like? Interested in fossils? Ithaca's Paleontogical Research Center and its affiliate Cayuga Nature Center may be able to provide you with information about the natural history of the area. Although the Nature Center is not open to the public (although the trails are), you can view the online exhibits such as the one created on Cayuga Lake.