Citizens United v. FEC
The Supreme Court has overturned the prohibition on corporate money in campaigns, holding that corporations have the same First Amendment rights as people do. They did not strike down the requirement of reporting those contributions.
The opinions in the case run 176 pages. It will take a while to digest it all. In the meanwhile, the opinion is here: http://www.supremecourtus.gov/opinions/09pdf/08-205.pdf, in case you want to read it yourself.