======= Date Modified 29 Mar 2011 04:40:49 =======
======= Date Modified 23 Mar 2011 03:59:29 =======
To block the whole Internet for a while you can use Freedom (http://www.macfreedom.com).

For blocking particular sites, Binary Switch Eclipse (http://blog.binaryswitch.com/2010/02/binaryswitch-eclipse.html) requires you to enter a long password to view those sites, and FocalFilter (http://www.focalfilter.com) will fully block the sites as long as you tell it to.

You might also be able to get around your need for IE with the IE Tab extension in Firefox:
http://ask.metafilter.com/80977/Can-anyone-recommend-a-decent-URL-blocker-for-IE-along-the-lines-of-Leechblock-or-somesuch

One more, there's a good Mac tool called Anti-Social (http://www.anti-social.cc) which has a Windows release upcoming.