Phycokey – University of New Hampshire
http://cfb.unh.edu/phycokey/phycokey.htm
Freshwater Algae in Lakes (State of Washington)
http://www.ecy.wa.gov/programs/wq/plants/algae/lakes/OtherFreshwaterAlgae.html
Varieties of Algae (with micrographs)
http://www.micrographia.com/specbiol/alg/alghome/alggen01.htm
A good, but fairly expensive, reference book to buy:
http://www.barnesandnoble.com/w/freshwater-algae-of-north-america-john-d-wehr/1111448643?ean=9780127415505