I believe that as a professional I must be accountable for how I spend my working hours. I should be able to answer questions like "How long did feature X take to implement?", "Why did Y take so long?" and "What have you been working on for the last week?". My memory is not always up to this challenge.
To solve this I wrote a very simple 'proactive time tracker'. It regularly pops up a message box to ask "What have you been doing for the last X minutes?" and logs the answers. This allows me to automatically record what I got up to in a day.
Another project trying to solve this same problem is: TimeSnapper
It takes regular snapshots of your screen (at configurable interval and resolution) and allows you to later play this back as a movie :) or jump to any point in time.
I reckon it's a nice solution. I am looking forward to replaying my week :)