SelfControl is a free and open-source application for Mac OS X (10.5 or above) that lets you block your own access to distracting websites, your mail servers, or anything else on the Internet. Just set a period of time to block for, add sites to your blacklist, and click "Start." Until that timer expires, you will be unable to access those sites--even if you restart your computer or delete the application.

Combine this small, easily customizable AppleScript with a keyboard shortcut utility (like FastScripts) to quickly toggle an application forward and back. I use this for Terminal and TaskPaper.

"Freedom is an application that disables networking on an Apple computer for up to eight hours at a time." Only restarting can circumvent the time limit you've sent.