The American Bar Association has an excellent web page that lists amici curiae briefs in U.S. Supreme Court cases that the Court has accepted. Here is the link to the ABA page for the case called Doe v Reed, 09-559, the case the Court will hear on April 28. Scroll down to the bottom to the April 28 entry.
Amici briefs in support of petition privacy have been filed by 47 organizations. There are 13 such briefs, because sometimes many organizations have joined together to file a single brief.
There is also an amici brief in support of neither side, filed jointly by the Brennan Center for Justice, the Center for Responsive Politics, and the Sunlight Foundation. It says, “The purpose of this submission by amici is not to urge the Court to reach a particular resolution of the claims before it, but rather to direct the Court’s attention to the broader disclosure interests that could be affected by its disposition of this case.” These three groups are neutral on whether petitions should be private, but they are urging the Court to make a distinction between privacy for petition signers, versus privacy for campaign contributors. Thanks to Peter Gemma for the link.
The only minor party that submitted an amicus brief is the Constitution Party, which signed on to one of the amici that speaks for many organizations.
Later there will undoubtedly be amici briefs on the side of Washington state. They aren’t due until mid-March.