These are known brands and companies who will be participating in the SOPA blackout strike. The ones with links actually link to the announcement from the site as to why they are doing the blackout to strike against SOPA.
Red 5 Studios
Center for Technology and Democracy
Electronic Frontier Foundation
and thousands more! Over 8,000+ websites have confirmed their SOPA blackout.
How you can get involved:
Tweet with #SOPA and #SOPASTRIKE