Here's a breakdown of some sites I use when looking around. One of my goals is to create a more comprehensive race calendar than what appears on Active.com, in part because there are a lot of smaller races that just don't make it there (and, with due respect to the Active folk, their event calendar is cluttered with running clubs/programs).
So, here's a list of some spots to check out in the interim. This is not an exhaustive list: it doesn't include single-race Web sites, and it's likely missing some running clubs that put out lists. Post some of your own or e-mail Brooklyn Running - hopefully this spring, I'll have a Web site up that is a bit more comprehensive.
New York Road Runners: http://www.nyrr.org/races/2008/calendar/march.asp
Prospect Park Track Club: http://www.pptc.org
Van Cortlandt Track Club: http://www.vctc.org/eventspage.html
Long Island Road Runners: http://www.lirrc.org/calendar.html
On Your Mark Productions: http://www.oymp.net/click_on_calendar.htm
Race Forum: http://www.raceforum.org/online2008.asp
Hit-Tek Racing: http://www.hitekracing.com/
Hartford Marathon Foundation: http://www.inghartfordmarathon.com/foundationnew.htm
General Race Information
Cool Running: http://www.coolrunning.comMarathon Guide: http://www.marathonguide.com