I don’t know how I came upon KeepMeOut, but it’s become a favorite productivity tool of mine.
You simply enter a URL of a site you tend to frequent too frequently (for me, it’s Twitter). Then you tell it how often you should be allowed to check back in.
KeepMeOut creates a URL that you can bookmark in your browser. Click on the bookmark, and it lets you through, but the clock starts ticking. When you go back before you’re supposed to, here’s what you see:
It’s easy to work around (just enter the URL of your forbidden site directly), but if you use it, it’ll help remind you that you’re supposed to be doing other things.