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). You can also set a password for access to the extension options, just to slow you down in moments of weakness! Sites to block can be specified using wildcards (e.g., *.somesite.com) and exceptions (e.g., +allowme.somesite.com).

LeechBlock also keeps track of the total amount of time you have spent browsing the sites in each block set.

Please see the homepage for more information: http://www.proginosko.com/leechblock.html