ExRx.net
Not flashy but a lot of fun and useful stuff. It describes a ton of home fitness tests, provides caculators that give population averages and percentile scores for things like pushup and sprint tests, and has a nifty instant-fitness-expert index of exercises organized by the muscles they target.
FitLinxx
Fitlinxx makes circuit training equipment that is linked into a central computer database to automatically record your workouts. I began using it at the Silver Spring Y. This Web site goes far beyond that, however, and offers a free (once you register) database to log cardio and strength training workouts. Data collection of this sort is important to track your progress. The Fitlinxx interface can be cumbersome, but other sites I have seen charge for what is basically a gussied-up Excel spreadsheet.
HealthierUS.gov
This is a main portal that the government has set up to encourage exercise and better eating. The information is entry level, with a lot of basic information and ideas on how to get started for people who have not been working out.
Mayo Clinic
The fitness section of the medical clinic is small, but has nicely presented articles that rotate, covering exercises to prepare for different seasonal sports and feature articles on issues like eating for performance.

Self Magazine Calculators
Desperate to find out how many calories you burned vacuuming? Find the answer here, along with tools for determining your BMI, ideal body weight and daily caloric needs.
Prevention¹s Walking Page
If you don¹t know where to get started, there¹s nothing easier than walking. But even though you¹ve been doing it since you were one, you can do it even better by following these tips.
Yoga Today
Slimming down your body doesn¹t mean you have to slim down your wallet, too. Download a free yoga class here every day.
American Council on Exercise Library
When you want to strengthen a body part -- but don¹t know how -- search for the right move here.