SOPAJS inserted this banner
SOPA stands for the "Stop Online Piracy Act". It's a bill introduced by Lamar Smith (R-TX) in the House of Representatives of the United States and is intended to help combat piracy. While most people believe in the value of protecting content owners' rights and content, many are concerned that SOPA will do more harm than good and are concerned that, through unintended consequences, SOPA will enable widespread censorship.
As usual, Wikipedia has an excellent summary here.
A PDF of Bill HR3261 is here.
Senator Ron Wyden (D-OR) strongly opposes the bill and issued the following press release.
The House of Representatives has a list here.
SOPAJS is a project to build a small, easy-to-use bit of Javascript that website owners can add to their webistes to display bits of information about SOPA. SOPAJS puts that "SOPA Supporters" banner on the top of the page.
If you'd like to add SOPAJS to your website, please see instructions here.