Ask HN: Time tracking tools?

I've always wanted to know how much time I actually spend on various work-related activities throughout the day, say programming, reading, studying, side projects and so on, but I've never actually bothered keeping track of such activities in detail. I think that it'd be at the very least interesting to find out how many hours I spent on something on a given week, and I'd like the approximation to be as precise as possible.

Are there tools out there that you use or have used that have allowed you to keep track of your hours in a fairly pain-free way and that you could keep up with as time went on? At one point at work we used to keep track of individual hours through pen and paper, but that gets real old real fast and you then have to still input your hours into a spreadsheet somewhere, an overall tedious process.

Thank you!