Google Search
 

I was checking some google rankings this morning and I am normaly 1st or 2nd page for a few terms and was ranked number 1 on all three that I was checking. I was like wow wtf. I got to look and it had showed how many times I was clicking it, checking my google cache and such. I looked and I was logged in from google gmail and it has been tracking my clicks and changing my rankings according to links I had clicked. I signed out and the search results where back to where they had been. Just noticed this, not sure if other people have noticed yet or sign into google for gmail and other apps. But every person that is signed into google will be getting unique search results based on what they have clicked in the past.

Depending on the options you set, you can remove and uprank results as well. If a lot of people remove or uprank a link, it may have some influence. You can also make comments on the links which turn the search results into a 'wiki' of sorts. Those comments appear to be viewable to others that are signed in.