Good job, Wally. It was tempting to ask things like Which U.S. airline was famous for its Hunt Breakfast flights or What was the first U.S. airline to serve Canada, Alaska, Mexico and Hawaii?. Do you remember the brand of complimentary champagne your airline used to serve? It wasn't very good but it was free.
As to answers, I don't count the cargo airlines or non-scheduled carriers such as Universal or ONA. We're only dealing with airlines that offered scheduled flights.
I remember AeroAmerica running scheduled service with 720s between BFI and GEG, but they never operated them up to Alaska unless it was a charter.
Alot of folks forget about obscure airlines that operated scheduled domestic service back in the 70s and 80s. Regular perusal of the OAG would yield all manner of gems in the early days of deregulation For example, for a short time back in the late 70s, Mackey International operated DC-8-51s between FLL and LAS. I was ticketed aboard an Arrow Air DC-8-62 once operting schedulked service (1985) between MIA and MCO. Sadly I missed that flight and never rescheduled.
Here's a bonus, just sitting here thinking off the top of my head: Name six U.S. airlines that operated the Fokker F.28-1000.