Where can I find really cheap airfare to seattle from denver?

Delta, United, and Frontier are all coming up $179 r/t total with 21 days or more advance purchase; ($191 currently with just 14 days advance purchase). This is a sale fare. It has fluctuated $170 to $199 since early August (see http://www.farecompare.com/fare-search/y… — click “historical deals”). You can also sign up there for sale alerts.

Southwest, which don't show up in other booking sites, cheapest currently is $79 each way, which comes to $179 r/t total with taxes and fees added. http://www.iflyswa.com

Ideal and cheapest to book direct from the airline in this case:

http://www.delta.com

http://www.frontier.com

http://www.ual.com