ATTQOTD: In addition to
www.runningintheusa.com a large number of races now use
Active.com for their registration, so it's a good place to look for upcoming events. For the 26.2 crowd, another good one to check out is
MarathonGuide. This site has been around forever so the historic race reviews are really helpful if you are targeting a destination race.
For local events, you can't go wrong with local running clubs and independently owned running stores. Also look out for regional race websites. Here in Dallas/Fort Worth there is
DFW Races which is a lot more granular than the nationwide race databases. So if I decide I want to run a 10K on a given weekend, I can usually find one in driving distance on that.
Word of mouth (and I include social media in this) can be extraordinarily helpful because you often are connecting with other folks wanting to do the same race.