MONKEYJUNKEY  Great! I love the idea of this. I also have the tendency to quickly visit my favorite sites while I should beresearching something else...  
(Version 1.2.1)

DRFUNKO  I'm using this on Mac OS X Tiger 10.4.11 and it works perfectly for me. (program version 1.2.1)

It's a very simple and effective program. If you've activated it, even if you try to quit it, the selected websites are still blocked. It can operate in blacklist mode, where you list sites you want to avoid, or it can operate in whitelist mode, where it blocks access to all sites except the ones you specify.  
(Version 1.2.1)

GOODWILL :)  I have been looking for such kind app for a while. I am the kind of person who always try to surf my favorite websites even when i should concentrate on more important work (such as writing a paper). This app help me a lot in that sense. One thing I would ask for the developer's favor is that to extend the block time from current 12 hrs to a longer period.  
(Version 1.2.1)

NMEGE  Doesn't work on my Mac. Entered parameters for one item in the blacklist, then clicked "start".

Got the following error message : "error -208. Could not copy Selfcontrol's helper binary to PriviledgedHelperTools directory".  
(Version 1.2.1)

DRFUNKO  I'm getting a problem where the whitelist feature doesn't work properly. If I set up a list of sites and select the "whitelist" option on the Domain Blacklist window, when I activate SelfControl my Internet access is cut off completely, even to the domains I whitelisted (Safari 4 continually says "Contacting 'domain.com'" in the status bar). The Internet does return after the set period of time. The blacklist feature works fine, though.

Personally, I'd prefer to use the whitelist mode, as it blocks out the majority of the internet, forcing me to only use certain on-task sites — but I guess if I blacklist the major search engines, that pretty much cuts me off too.  
(Version 1.2.1)

THE VALRUS  Wouldn't it be better to have a whitelist?  
(Version 1.1)

STOPDISTRACTIONS  Check out http://www.StopDistractions.com — StopDistractions blocks access to distracting websites like Facebook, MySpace, Twitter, and Digg. You can personalize the list of blocked sites and you can set a countdown timer. StopDistractions runs on Windows XP or Vista.   
(Version 1.2.1)