Another tracking idea I once had: a browser plugin that tracks what web pages know about you. For example, if you visit web page A and afterwards web page B, and both pages use Google Analytics, the plugin would tell you that Google now knows that you surfed from A to B. If you then login to B with your user name, the plugin would tell you that Google now knows your name. And so on (not specific to Google, there are lots of services that span multiple pages, typically advertising). It would be interesting to see how web pages cluster together according to such "spanning" services.