Do you reckon that you're spending too much time on a particular website such as Facebook, Twitter, or something else? Do you want to reduce the amount of time you spend on that site, but you're not quite ready to install software which permanently blocks you in a way that is difficult to work around?
If so, check out Keep Me Out (www.keepmeout.com). Give it the name of a site, and the number of minutes to block you for, and it'll create you a special shortcut to place in your bookmarks bar or on your desktop. If you access the forbidden site more than once in the allotted time, you'll receive a friendly warning not to go there. You can still visit via the original URL, of course, so this isn't exactly a foolproof method, but it will hopefully assist in getting you to cut down your visits. And a helpful graph even shows you how naughty you've been, ie how many times you've attempted to visit a site before the time limit was up.
Keep Me Out is free, and purely browser-based so there's nothing to download or install. It's great for imposing a bit of discipline on younger internet users for whom the workaround might not be so obvious. For the rest of us, it's some harmless fun.