air fare sites

try flights.google.com
Enter Denver as the departing airport, enter dates and click on any spot on the map. You have to enter dates because the price varies by date and length of stay.

You can go to skyscanner.com and type in DEN as your airport and Everywhere as your destination to see where you can go and how much it might cost. I see you posted this question in the Spain forum so if that is where you really want to go you would type in Spain instead of Everywhere. If you find something you like you'd go directly to the airline's website to book.

Are you looking for direct flights only? Otherwise you can get anywhere...

Go to Wikipedia and put the name of your airport in. It's going to tell you the entire history of that airport and everything about it. Many of the airports' history is quite interesting.
Wikipedia lists all direct (non-stop) flights into and out of that airport; It's a real eye opener. Some airports have few carriers going to many places. Some airports have a bunch of air carriers with just a few flights. Some airports in pretty large cities have relatively small airports and service out of them. And just about every airport in the world is in Wikipedia.
I don't know who maintains the list of airlines and cities flying out of airports, but it's upgraded daily and is very accurate information. Wikipedia also lists all the airlines' websites so you can go directly to them to check pricing.

Thank you! I think sky scanner is what I was looking for!

Your airport's information desk or website should also be able to help you.