Some great programs similar to LeechBlock
I love the Firefox LeechBlock tool but sometimes I want to block Internet completely rather than just block Firefox. I have had good results with a $10 tool called Freedom (http://macfreedom.com/) which is available for both Windows and Mac.
There's also a completely free program for Mac called Self-Control (http://eyebeam.org/projects/selfcontrol). It can be used to block Internet completely, or to block just websites you choose to block (you can also selectively block chosen kinds of non-web Internet use).
The same site that sells Freedom also sells a $10 tool called Anti-Social which blocks various time-wasting websites in all browsers, while letting other websites through. Currently it's Mac only but the author wants to make a Windows version as well.
I learned about these programs in this article which is quite interesting:
With the Chrome browser, you can use StayFocusd or Chrome Nanny.
EDITED: To add StayFocusd and Chrome Nanny, and clarify that LeechBlock is a Firefox add-on.