TimeSnapper advertises itself as an "automatic screenshot journal", which means that takes periodic screenshots of whatever's on your screen and then lets you play them back however you like. You can configure the number of seconds between screenshots and the playback mode lets you pick a date and then use a slider/timeline widget to show a particular part of the day and play it back at whatever speed you want. It's sort of time lapse photography for your desktop, and could be handy for figuring out, for example, how much time you spent on Project X. Best of all, TimeSnapper works on multiple monitor setups and is completely free.