I really like the idea of tracking myself (e.g. books read, weight, steps taken etc.), but as a hacker I'd never let anyone else hold this data. I mean, Facebook is one thing, but highly personal details about my personality is another thing.

That said - it would be cool to see something like this open sources. Currently I'm using excel to track most of the stuff I'm interested in, but I know that there must be a better alternative.