I spent a good deal of effort 6 years ago to find an alternative in Denver, CO, that wasn't Comcast or CenturyLink. There are a number of business ISPs, but only a one or two residential ISPs. Verso Networks is what I got. Excellent service, but they were geographically limited.
In the suburbs of Denver, you have a choice of Comcast or CenturyLink if you're lucky.