I've come across a few different applications that monitor my usage while on the computer, but what have you used, and like? whether it be writing down your activities in a composition notbook or install an app that reports silently to a server? what do you like?
For explicit time tracking for projects I use SlimTimer. I also run RescueTime to track my time implicitly, but I end up not looking at that data very often.
I use neomem to keep track of things by hand. This allows me to keep track of more than just time. I often keeps notes associated with the story. This has become a collection of knowledge that I often search.
I prefer writing the times down using pen and paper. That way you can more fairly weigh things that would have been miscalculated if you were recording them with a stopwatch or timer.
If you start on something and have to get up for a few minutes, a timer may count that toward your working time had you neglected to stop or pause the timer. The good-old pen and paper are going to more accurately show which tasks you focused most of your time and energy on...not just the ones that you started earliest and ended latest. It may not be 100% accurate, but neither is the timer if you don't use it properly.
I have used both in the past, and find that there are problems with both, but I prefer the pen and paper method.
We use Standard Time as a Time Tracking tool and it's got a nice quick tasks window that is relatively small and lets you easily click checkboxes to switch between tasks. You can also create projects/customers and pre-set tasks to be loaded for each project (development, unit testing, documentation, etc...) and then just use the quick tasks window to switch which task you are currently working on without wasting too much time going through a full blown GUI.
It's not cheap - about $150/user - so if it's for personal use it might not be the best bet, but if you're looking for a solution for a team of developers then I've found it to be a good fit.
I use a paper timesheet to track my time. It looks like this:
and as a back up, I use a paid version the excellent TimeSnapper in case I need to go back and retrace anything I missed tracking the time on.
I use the time tracker plugin on Firefox. It tracks my surfing time of the whole day