This is an online database hosted by the USDA which is a collaboration between US and Israeli scientists. It includes classification, nomenclatural history, distribution, hosts, and literature information. It's a good source if you have a scientific name and want authoritative information.

There are a variety of USDA resources in the menu that vary in usefulness and should be dealt with separately, including a photo database, a list of the nearctic taxa (thanks, =v=) and an interactive key to families.