ultrunr.com: Ultra running resource, with all the tips/advice/experience/information on running a Ultra
run100s.com: List of all 100 milers in US
ZombieRunner.com: Online store for running related products, predominantly ultra running.
Multidays.com: Site with all calendar of multiday races, training tips.
Running Injury related websites:
Courtesy: Coach Rajeevtherunner.