Old fashion, but I use SkyGuide (or OAG) to look up my city and an airlines major hubs and see how many flights they have and when.

Also, you can download timetables for AA/UA/CO and plug in your city, then select "TO: ALL CITIES" and see all the flights that airline operates. Only works for those three airlines, since no other airline has a timetable that can do such a function (NW/AS/others have e-timetables, but you can't do an all-city search).

Most airport websites list all the airlines that fly there, and many additionally list the airlines destinations and frequencies.

The back of Amex SkyGuide list all the airlines that fly to a certain airport.