I'm a fan of quantified self, but I'm not comfortable sharing this data with yet-another third party service. Tracking email, browsing habits, application usage, etc. is valuable, but the collected information should stay on my laptop.
[edit]: Cozy is a self-hosted web application (similar to ownCloud) so you're sharing the data with yourself. My point about browsing history and application usage still stands.
About six months ago I wrote an OS X app (and browser extension) that tracked all my computer usage. It never grew past a prototype, but I'm thinking of starting work on it again.