1. Ultrarunner.net
THE BEST THING about this website for Ultrarunners in the United States is the long lists of races organized by region. I was disappointed at first to see that the Race Listings page hadn’t been updated since last year, but I found 2011 updates after testing the info buttons for a few races with 2010 dates.
Ultrarunner also hosts a series of races in Northern California and Nevada, including 10K, 50-mile, 12-hour, and 24-hour races.