For work I have weekly text files, where I write down what I'm doing. Originally this was to help me compile the weekly timesheets that my (then) employer requ

For work I have weekly text files, where I write down what I'm doing. Originally this was to help me compile the weekly timesheets that my (then) employer required. But the notes saved me from a lawsuit and I know find it useful for keeping notes on why I'm building things a particular way (especially when coupled with grep/ack/Spotlight). For personal stuff, I have an A5 hardback book and a fountain pen - nothing beats it.