What kind of spider is this? Is it a poisonous spider?
Arapahoe County Colorado spiders
I think this is a funnel weaver (or funnel-web) spider. See http://bspm.agsci.colostate.edu/files/2013/03/Funnel-Weaver-Spiders1.pdf for details. These are not considered to be medically important spiders, although, as this publication indicates, there is some confusion in this regard.
The black house spider is an Australian species and is not known to occur in Colorado. The funnel-web spiders also are in a different family. One of the species in the funnel-web family is known as the domestic house spider, which may be a source of confusion.
That said, it is extremely difficult to identify spider species from photographs, so the only way to be sure would be to have the specimen identified by an expert.
So is the funnel web spider poisonous?
There are a number of species in the funnel-web family and none are considered medically significant. There is a report, now discredited, indicating that the hobo spider, a funnel web spider, could cause slow healing wounds. Also, because of where they are found and their general coloration, these spiders are often mistaken for the medically significant brown recluse spider. The brown recluse does not occur naturally in Colorado, although it has been collected a few times from household effects recently brought into the state.
The medical importance of funnel-web spiders is discussed in more detail in http://bspm.agsci.colostate.edu/files/2013/03/Funnel-Weaver-Spiders1.pdf.