This site has a good race calendar, particularly for Colorado and the western states.
Results are much less comprehensive.
The site has potentially useful listings of trails by state, although the quality varies substantially and broken links are common. Running This site is a focus for trail running in New England. It even posts every runner's cumulative score as the series progresses through the year. Nevertheless, it is a good site for obtaining lots of information on trail running and the very long trail races. Trail Running Tips Particularly when the ground is damp, avoid stepping on rocks, roots, or anything other than the ground.