quick and simple: edit /etc/hosts
(or on Windows machines: C:\WINDOWS\system32\drivers\etc\hosts)
and add the following:

0.0.0.0 www.facebook.com facebook.com apps.facebook.com
0.0.0.0 www.myspace.com myspace.com
0.0.0.0 www.digg.com digg.com

etc. Add as many sites as you want. When you want to stop being productive at work, simply comment out those lines (like during a lunch break). Just be sure to uncomment the lines when you need to be productive again…

facebook, productivity