I wrote about this on my blog the other day:
Increasing your productivity with a time log.
Despite being a computer nerd, I find a paper system to be much easier to maintain. It can be carried around and can be scribbled on for notes etc. The other advantage is that having a (somewhat) rigid structure keeps me from spending time messing around on writing scripts or whatever to maintain a log.
The results over the years from using a time-log have been extremely impressive. It helped me to concentrate on a single task, and it made me realise just how much time I wasted on the 'net. I try not to be too detailed with what I'm doing, and if I'm going to be doing a lot of a little tasks, I'll write something like "working on x's todo list". I find that adding too much detail eventually becomes counter-productive as you end up spending more time logging than doing.