A friend of mine pointed out a very cool Firefox add-in called MeeTimer that I really think is a good idea. If you've ever spent waaay too much time on YouTube after someone sent you a link to a video, you should check this out. Same goes for getting sucked into web "research" that causes hours to go by without you noticing.
What's MeeTime do? Well, I think they describe it pretty well on their site:
And, if reporting isn't enough for you, you can configure MeeTimer to popup a warning dialog when you spend too much time on a particular site.
Like most Firefox add-ins it's free, though the author does accept donations if you find that this tool is useful (and you probably will). If you're a Firefox user and want to reduce the time you waste online, install MeeTimer right now.