Fitness, health, diet science
This website is directed by Lyle McDonald, very well known in the fitness community.
Where I got my body fat tested
This website is great for tracking calories. They have a H U G E database of food. I would just highly recommend avoiding the forums on here.
Another fitness genius who analyzes scientific topics on fitness for the lay-person.
Probability & Statistics
You can learn anything on this website. Quite possibly the best online self-study resource I've ever come across.