It's Just Running
This is the most comprehensive list of races in the United States from 5Ks through ultras.
Very old school site but if you are interested in 100 mile races in the United States, this is your go-to resource.
The calendar on ultrarunning.com is fairly comprehensive. Note that it lists difficulty by elevation (1-5) and terrain (1-5) as reported by each race's RD.
Ultrasignup is the place you'll be going to sign up for most ultras. It has an associated calendar.
The granddaddy of race calendars. Marathon-specific, and mostly road-specific. Also has reviews. Their design hasn't changed since about 1880, but the site is very useful.
This site is a bit like Marathon Guide, but focused on halves. They have added a bunch of other content over the past couple years, and you have to be careful about clicking on calendar-looking items that are really ads, but a great calendar is buried in there.
Tracking and Logging
It is easier to track your progress and know where you are going if you can tell where you've been.
The best tracking, graphing, and social tool on the Internet.
One of the originals remains the state of the art for logging, graphing, and analyzing your running history. The site also contains a nice pace calculator and other tools.
Race Reviews and Reports
If you are an ultrarunner, Bibbz is a fun place to go find (and post! please post!) race reports from a variety of races. It's still gaining critical mass, but it gets a big thumbs up from me.
Race Reviews. Not all 'raves'. Also has a calendar.
Kind of like Race Raves, but not.
Once upon a time, Ultrarunning Magazine was a printed collection of race reports, awesome photos, and results. Much of this has been transported online. The race reports section is extensive, but be prepared: it can be hard to find what you want.