For January 18, 2012 many websites have participated in the protest to oppose the Stop Online Piracy Act & Protect IP Act. The websites I've discovered are: Reddit, Imgur, Wikipedia, Google, HypeM, and there is probably thousands of others.
First up is Hype Machine. A music oriented website, that indexes music blogs. Many artists end up here, being discovered, shared, and sales generated. I can't speak directly for the sales, but I can imagine they pick up a heavy fan base. If SOPA was in-affect, this website would most likely not exist.
Next, Google. An arguably powerful search engine that has served billions of queries to billions of key strokes ('er, humans). The Google team wishes to index and display the web as it is, and not how we want it. Again, SOPA would destroy the structure of any search engine, not just Google's.
Wikipedia. A digital user-based encyclopedia. A free one at that. Used by millions, and edited by millions. Wikipedia serves pages about history, and current events, ranging multiple subjects. While they didn't pick out bits of the website to be blacked, they did lay-over a big notice. SOPA would be in huge conflict with the linkage.
GoDaddy, once siding with SOPA has changed their minds. They are now opposing it. If I could make a guess, I would say it is because they started losing some customers (money) over it, or benefit of the doubt they have realised how damaging the bill is.
NameCheap has been anti-SOPA.There's been several people I know that have been migrating their domains to NameCheap (affiliated) or another domain registrar of their choice.
The flip-side of SOPA, and cybertised protesting, other websites have not been promoting the Stop Online Piracy Act. Almost as if it's non-existent.
Yahoo, Microsoft, AOL, Twitter have not promoted any of SOPA, however SOPA is on the Trending list of Yahoo, as-well as various hashtags on Twitter.
People have been going on about this for a year or more, but in the last week it has been flared up consistently. This is good, due to the awareness it'll hopefully receive more bad attention, and shifted out for the already existent Digital Millennium Copyright Act.
Update, January 19, 2012: According to Ars Technica, eighteen senators now oppose PIPA. Read more here: http://arstechnica.com/tech-policy/news/2012/01/pipa-support-collapses-with-13-new-opponents-in-senate.ars