Bing, the world's 2nd best used general search engine (after Google) has just announced the release of adaptive search. A brief overview can be seen via this searchengineland blog article.
Adaptive search excites me enormously and it's something we're working on at the moment. I say working on, we're working on the theory and how it might work. We need to save up our pennies and test a few assumptions before building it!
The principle is that adaptive search learns the type of person you are and alters the results based on what it has learnt about you. This means different people would see different results. For instance, a general practitioner in the UK might search for hypertension and s/he would see different results to a cardiologist based in Canada. It makes sense that they see different results as their contexts are different.
We're actually pretty confident with the theory but the biggest assumption is, for this to work, people need to login. So, we're exploring that and allowing people to login with their Facebook and Twitter accounts should make this easier.