What timekeeping software do you use?

Looking for an ultra-lightweight & economical way to record time spent on client work...especially want a way to click a button on my Windows Taskbar at the start of a client-related phone call.

Bonus points for an iPhone App for those times away from desk.

Not looking for feature-rich (and very expensive!) products like TimeSlips, nor looking to have to keep QuickBooks open all the time.

Welcome your suggestions with appreciation.