Northern false widow Steatoda borealis
This northern false widow was momentarily stunned after a surprise encounter with a yellow sac spider, which ran directly into it but then continued on its way without further incident.
Spiders in the genus Steatoda have the misfortune of being usually dark in color, and more or less globular in shape. This is often enough to confuse them with the dangerous Latrodectus spiders, or widows. The northern false widow is a very small spider that can often be found indoors, especially in winter. Inside they are one of many species responsible for the kind of untidy webs that get called cobwebs. These traps are for capturing very small flies and moths, though larger Steatoda spiders are known for preying on other spiders, including true black widows.
Spiders of the North Woods, by Larry Weber. It's not perfect or complete, but it's the best book for spiders in northeast North America I've found.
Bug guide dot net is a great resource for looking at photographs of insects and spiders and comparing them to your own.
A dichotomous key for advanced identification of comb-footed spiders.
The entry on this spider from the Nearctic Spider Database.
The woodland park zoo describes the west coast relative of the northern false widow.