I used this for a while approximately 15 years ago. It was excellent and while I was composing an email it would find things in my history that were useful to the conversation. Alas, I can no longer do all email and web browsing in Emacs. I really wish such a thing existed for Mail.app in OS X.
It's to bad that this doesn't run as a daemon that can simply be queried through an API. Back in the day I remember a linux project called ... dashboard? (found it! http://nat.org/dashboard/) That seemed like a great tool. For whatever it got scrapped and it's guts turned into Beagle. Since I no longer use linux on the desktop it's hard for me to evaluate, but it looks like it's now a tool similar to OSX's Spotlight.
