Active.com is a great place I've learned of to find more information about and register for triathlons. For example, you can register for The Nation's Triathlon in Washington, D.C. now from the site.
Active.com's offerings aren't limited to Triathlons. You can find out about events for other sports as well. And Active.com is also a great source for information and tips that will help keep you at the top of your game.