Cold Turkey is an interesting tool which makes it easy to prevent your own access to particular websites.
Why bother? If there's work to be done then you may not have the time to visit Facebook, check Twitter or post in your favourite online forums. If you're strong willed and can simply ignore your usual web destinations, then this isn't a problem. If you find yourself sneaking back to particular sites, though, and wasting time you don't really have, then using Cold Turkey to control your online access could be a welcome alternative.
The program is easy to use. At a minimum you could enter facebook.com and a few other domains, click Start, and you'll be prevented accessing those sites for an hour.
If the time limit isn't suitable then you're able to choose when the block should end on the Timer tab (up to a 3 day maximum).
Sites can also be organised into groups, each with their own schedule, so you're able to restrict the real time-sucking sites more heavily than others.
A $19 Pro version extends the package with support for wildcards (facebook.*), exceptions, full scheduling of block times, the lifting of the 3-day maximum block, and the ability to restrict access to selected applications, too.