Evidence-based medicine. While most docs now are learning about the principles of EBM, there are very few resources available that lead the way. Bandolier has always been one of my favorite sites. Mixed with a bit of humor (or should I say humour!) and wonderfully written analyses of many common clinical problems, this is a resource that all physicians should know of.
Another real winner .. though for different reasons .. is Brian Alper's Dynamed site. Dynamed is an onlie resource with good, clear clinical synopses of over 2000 conditions. You have to log in and set up an account, but access is free.