Frontier Airlines flies between: Denver - Las Vegas, Detroit - Denver, Chicago - Denver, Seattle - Denver, Denver - New York, Denver - Chicago, Atlanta - Denver, Dallas - Denver, Denver - Phoenix, Denver - Los Angeles, Houston - Denver, Denver - Orlando, New York - Denver, Denver - Atlanta, Phoenix - Denver, Denver - San Diego, Orlando - Denver, Tampa - Denver, Denver - San Francisco, Denver - Seattle, as well as several other routes.
If you're flying on Frontier, you will almost always have to pay for your checked baggage. Make sure to book and pay for your luggage online prior to arriving at the airport as you'll end up paying less. Both your first and second bags are $20 online or $25 at the airport. Any more than two bags will cost $50 each. Note that if you are flying in Classic, Classic Plus and Summit/Ascent Class, you are entitled to two checked bags free of charge.