Best for Finding a Race: RunningintheUSA.com lets you search for races in specific states, by distance and date. It has the most comprehensive list of races.
Runner-Up: Mychicagoathlete.com lists everything going on in the Chicagoland area. It is, by far, the best Chicago running, triathlon, and other sports race webite. All areas of the country should have a website like this.
Best for Information on the Boston Marathon: This page on CoolRunnning has answered just about every question I had.
Best Race Results Website: Athlinks wins by far. The systematic part of me loves how you can easily bring up any distance you've run, or look at races in certain years and even how you placed in races. I also enjoy the fact that you can look up and see how other people have done. I may or may not have spent time looking at other results....
Runner-Up: marathonguide.com lets you search for your marathon results by year. They will tell you if your race was a BQ or not, too.
These are just a few of my favorite websites. Do you have a favorite?