Identification, Images, & Information
For Insects, Spiders & Their Kin
For the United States & Canada
Includes links to keys for several families of flies. (This link is also listed under the Internet references in the order diptera, but it seemed worth repeating here.)
http://www.hr-rna.com/RNA/Other%20insect%20pages/Key%20links%20page.htm

hmmm...
not sure how to get from that title page to the actual keys --something's not right there, must ask Herschel...
anyway, here's Herschel's version of the Weems (1953) Key to the Syrphidae of the southeastern US

For those of us west of the Rockies
please note that most of these keys are designed for Eastern US species.