Here is search logs of 650,000 AOL users. It's very interesting to view search history of particular person and analyze his personality. Let's do it together! Read more about AOL search database scandal or view research papers on web searching.
Push-Down Automata and Context-Free Grammars
Lecture notes defining context free grammars and closure and decidability properties of context free languages. There is a short section showing that natural languages are not context free.