Ask HN: Most unobtrusive time tracker?

I want to start seriously measuring my productivity, or time on task. I'm running OSX. I have an iPod Touch and an Android on my desk as well. Between those platforms, what is the most unobtrusive time tracking application? (e.g. hit button start work, take a break hit button to pause time etc) In addition to unobtrusiveness, it would also be useful if it kept a tagged/labeled history of the time for me (otherwise, I suppose a simple stop watch would suffice).

Any suggestions?