About six weeks ago I started using TagTime, a time tracker that pings you at random times. One big advantage of TagTime is that it integrates seamlessly with Beeminder and makes it feasible to beemind lots of things with minimal overhead. (Though if you just want to track productivity and work mostly at a computer, you're better served with RescueTime's Beeminder integration.)

I've also found it informative (and quite pleasantly reinforcing) to plot the general trend of how much time I spend on various things such as work, school, sleep, commuting, standing vs sitting, and so on. I use an R script for that, and have an interactive app for a few time use categories here (sample code here). I've given the link to a few friends, which so far seems to help me make better choices.