AphID - An identification key to the world's most polyphagous and/or cosmopolitan aphids
Aphis of the midwestern USA - 3I interactive key
Aphids on the World's Plants - Blackman & Eastop's seminal works, kept up to date in digital form
Aphids in life
Aphid identification and information at Influential Points, with a focus on the European fauna
Aphids of Quebec - Species list, photographs, predators, parasites, references (in French)
PhylAphidB@se for the identification of European aphids.