Ask HN: What do you use for time tracking

As a freelancer, its important that I accurately track and bill my time. I think a lot of us end up guessing about time on projects and tend to under-bill.

What do you use for time tracking?

(I am writing an app that 'solves' time tracking in a new way and am trying to determine which services I should target for integration. Currently supporting Harvest, what else is important?)

Please post the name of your time tracking weapon of choice so we can see whats popular.