Identification, Images, & Information
For Insects, Spiders & Their Kin
For the United States & Canada
As of May, 2011, the USDA Diptera site has been closed. (A link back to BugGuide is provided so that this note can remain posted.)
http://www.bugguide.net/

not too dead
some exceedingly useful BDWD fragments linger, e.g. the stats summary