Diabetes 

Blood Sugar 101 (one of the very, very few diabetes web sites which is not written on the assumption that you are a dope).

American Diabetes Association (check out their "diabetes news" and "diabetes research summaries").

dLife (useful information of many kinds, and a very useful and supportive discussion forum).

National Diabetes Information Clearinghouse (mainly a clearninghouse for brief summaries, of the kind which leave me unsatisfied but could be just right for people less obssessive than myself).

Canadian Diabetes Assocation (check out their "Professionals" section, and pretend you're a doctor -- but be aware that medical tests use different units of measure outside the US!).

Center for Disease Control/Diabetes (your tax dollars at work).

LifeScan (a commercial website, but with some useful information).

Fitness  

Use these links for advice on fitness, and also to locate fitness events (such as hikes, footraces, and bike rides) happening in your area.

Active.com

FitnessOnline  

Fitness.com  

DietDetective

Living well is the best revenge

Whole Foods Market  A commercial site, obviously, and not all of the information it provides is going to be worth your time. But there is a lot of useful information about various foods (including nutrition facts, cooking tips, and recipes).

Post Punk Kitchen   Isa Chandra Moskowitz and Terry Hope Romero make vegetarian cooking fun. (Their list of 10 Things That Are Punker Than Anthony Bourdain includes "the National Weather Service".)
 
Wine Library TV  Not everybody with diabetes can get away with drinking wine, but if you can, you might as well enjoy the good stuff. (Choosing quality over quantity is a pretty good strategy for diabetes patients, and not just in relation to wine.) Gary Vaynerchuk's daily wine-review videos provide a useful resource for learning about fine wine, but to say that they are "lively" is putting it mildly: turn down the sound before visiting his site.