LeechBlock is a free Firefox extension which enables users to prevent or restrict browser access to specific websites.
The system offers a lot of control over its actions. If blocking twitter.com forever seems too much, you can alternatively limit access during one or more time periods (0900-1230, 1330-1700), or ration access to the site (allow 5 minutes in every 30, say), and apply these restrictions only on defined days of the week (Monday-Friday).
There's some control over the page that's displayed when you try to visit a blocked site.
You can even set up multiple blocking sets, perhaps giving social networking sites a different set of rules and restrictions to gaming sites.
LeechBlock is a well-designed and supremely configurable tool which makes it easy to limit access to your most time-sucking web distractions.
Just keep in mind that it can only control site access in Firefox, and so isn't suitable as a parental controls system. If someone's determined to break the rules then there's nothing to stop them firing up some other browser and going wherever they like.