LeechBlock is a simple free productivity tool designed to block those time-wasting sites that can suck the life out of your working day. All you need to do is specify which sites to block and when to block them.
You can specify up to six sets of sites to block, with different times and days for each set. You can block sites within fixed time periods (e.g., between 9am and 5pm), after a time limit (e.g., 10 minutes in every hour), or with a combination of time periods and time limit (e.g., 10 minutes in every hour between 9am and 5pm). More Info »
With the lockdown feature, you can block sites immediately for a specified duration.
You can also set a password or random access code for the options page, just to slow you down in moments of weakness!
The sites to block can be specified using wildcards (e.g., *.somesite.com) and exceptions (e.g., +allowme.somesite.com).