I edit my hosts file (Windows: /Windows/System32/drivers/etc/hosts, Mac: /etc/hosts) to look something similar to:
127.0.0.1 news.ycombinator.com
127.0.0.1 techcrunch.com www.techcrunch.com
127.0.0.1 reddit.com www.reddit.com
etc.
Alternatively you can use other tools to block traffic. RescueTime (http://www.rescuetime.com/)is an awesome tool as well.