Personally, I use Twitter to keep my activity log. I’ve seen a number of different apps for helping keep track of how much time you spend on this or that activity, but Twitter is dead simple and thanks to its IM/SMS features, it is also ubiquitous.
First, go to Twitter and sign up for an account. Second, follow the instructions to activate Twitter for your IM and cell phone. Then, as you go throughout your day, simply type in short updates about what activity you are working on at the moment.
I almost always access Twitter through IM, so when I get to my office I might spend an hour checking and responding to e-mail. I’ll just type in “checking e-mail” into my IM. Then when I switch to working on a project, I’ll just take a couple seconds to switch to the IM app and type “Working on Project X.”
Updating in this way through an IM client or via SMS takes almost no time at all and the result is a relatively fine grained look at how much time I spent working on various things throughout the day.
The one thing I don’t do, obviously, is actually share my Twitter activity with anyone (which isn’t a big loss since Twitter’s RSS features seem not to work anyway, at least with Google Reader).
I save the Twitter archive page on a weekly basis so I preserve that information permanently, and then delete all of the updates on Twitter so I’m keeping the amount of information actually stored on Twitter to a minimum.