I use Flight Aware to track flights. It tracks airline flights as well as any flight with an IFR flight plan. (I don’t think it tracks VFR flight plans, but I could be wrong.)
Another site that I recently found is Flight Stats. It seems to track just airline flights.
The FAA has a site that tracks flight delays.
flightradar24 shows all of the flights in the air in the US and in many other parts of the world as well. You can search for an airline flights by flight number to find current and scheduled flights. It also shows private planes in the air and if you think someone is in the air, you can search by N number.