Certainly Porter Airlines falls under this designation: they fly from a secondary airport (which is very rare in Canada), the Billy Bishop Toronto City Airport. Also, instead of flying into Detroit airport they fly to Windsor -- there are only two non-seasonal scheduled flights from Windsor. Three from tomorrow because Porter adds a flight to Montréal-Trudeau. That Windsor-Montréal-Trudeau flight will be the first flight of Porter's not using the Billy Bishop airport. So while Porter appears in for eg. Kayak searches you need to know what to search for.
Whether you count Windsor as US is debatable: while it is certainly on Canadian soil, there is public transit into Detroit from there. The only caveat being the bus #8 from the Windsor airport arrives to the Downtown Windsor Terminal minutes after the Tunnelbus leaves for Detroit (both buses run every 30 minutes) so you'll have to wait almost half an hour there. But if you can rent a car it makes little difference whether you land at DTW or Windsor. Arguably it'll be a hell lot faster to get out of the Windsor airport, too.
But Chicago, Pittsburgh, Burlington, Boston, Newark, Washington DC are definitely USA.