Track useGoogle lets you keep a detailed history of your time on the web . . . which comes in handy when you're trying to figure out why you didn't get anything done last Friday, and at least part of the reason is that you checked 172 web-sites after lunch. And just to keep me honest, I've asked my wife to check this on a weekly basis.
Time useI keep a stopwatch by my computer. When I'm on the web, I start it. When I'm off the web, I stop it. At the end of the day, I clear it. And no, I don't do this in an effort to limit my time -- I tried that, and it didn't work. But just knowing how the time adds up does help me to control it.
I also keep a kitchen-timer by my computer --one that's preset to go off in ten minutes. When I go on the web, I start it. When it beeps, I reset it and keep working until it beeps again in ten minutes . . . at which point I usually say, "Okay, that's enough."
Schedule useIt's tempting to check favorite websites every 15-minutes or so, just to see if there's been an update. To avoid this, I've set up a "Daily Reading" folder for bookmarks to the sites I check daily (viz. The New York Times or Facebook), and a "Weekend Reading" folder for bookmarks to the sites I check Sunday mornings (viz. Lifehacker or Cool Tools). And yes, I suppose that I could still check those sites every 15-minutes if I wanted to . . . but just opening the folder is usually all it takes to make me pause and say, "not today."