TagTime is a time-tracking app originally developed by the same folks behind Beeminder. It asks you what you're doing randomly throughout the day. A small but passionate contingent of Beeminder users use TagTime to track hours spent on projects (and/or their entire lives), so you might hear about it if you lurk the forums or read the beemails!
If you're interested in using it as a Beeminder autodata source, check out the link above for more information and setup instructions (warning: it is not a one-click install) and feel free to contact support if you need any extra help!
There's also a TagTime Android app that's less powerful but much friendlier.
If you're interested in beeminding your time, but not so interested in the randomness of TagTime or the frustration of getting it working, check out the RescueTime integration! If you already use a time-tracking service (e.g., Toggl, Harvest), an IFTTT or Zapier hookup to a Beeminder goal might do the trick.