There are many open source communities which are protesting the anti-freedom bill SOPA and its 'Little Boy' cousin PIPA via their websites. Here are the top 5 open source communities protesting SOPA and PIPA.

openSUSE:
When you visit the opensuse.org site it redirects you to stop sopa page, which also says that openSUSE is on strike today: "The page http://www.opensuse.org/en/ is on strike today to fight SOPA and PIPA."

Fedora
The main slide on the Fedora home page flashes a message against a black background which says:
Stop Internet censorship. The Fedora Project does not promote Internet censorship. Help stop SOPA and PIPA.

Mageia
Mandriva Fork Mageia is also participating in the protest by blacking out its home page.

Mozilla
Mozilla is one of the leading protesters of SOPA/PIPA. The page will blackout showing this message to the visitors.

WordPress
The open source blogging platform WordPress is also voicing out its SOPA/PIP protest. There is a special plugin which allows WordPress sites to participate in the blackout.

Not participating: Dude Where Is Linux For Human Beings?
We were surprised to see that the most popular and the most vocal open source community Ubuntu did not participate in the anti-SOPA/PIPA protest. Not only the site has not blacked out, but also there was no statement by the Ubuntu team about the bill.

Being one of the most popular GNU/Linux based distributions Ubuntu could have done an excellent job of informing its users of the dangerous bill. I was expecting the same riot against SOPA/PIPA as I saw when Linux Mint topped the DistroWatch charts.

Other open source communities which did not participate in the blackout or released any statement regarding SOPA/PIPA also include popular CMS platforms such as Drupal and Joomla.