I just use iCal… that way I can block off parts of my day ahead of time for scheduling and then adjust it as the day pans out.
Then when the projects done I just go back through and tot up my hours… I also have a clear view of the projects I have worked on in any one week.
I tried using a simple timer but I would always forget to start it or stop it… and I generally round things up to the nearly 15 mins anyway.
A vote for Freshbooks too… I do all my billing using it, I find it very good indeed. I used to do all my expenses too but it started getting a bit too much for me, never finding the time or really wanting to enter all the data so I got myself a bookkeeper for all that fun stuff.