The National Education Association Advocacy Fund, a Super PAC which has spent $4.2 million to influence the 2010 election cycle so far, receives all of it's funding from the National Education Association, a labor union—or a 501c6—and also its parent organization. No individual donors are listed.
While one can probably safely assume the NEA's money comes from members dues, this is an example of the ability influential nonprofit groups have to completely hide who funds their political agendas. In cases like this, money is shifted from the parent group to its advocacy fund or action fund, and due to rules allowing 501c4s and 501c6s to not disclose their donors*, funders are hidden in the shadows.
See the the group's disclosure report below.
*Reporter note: I originally incorrectly reported this group as a 501c6. According to the group's form 990 it is 501c9. That is a tax-exempt group also called a Voluntary Employee Beneficiary Association according to the Internal Revenue Service. Nonetheless, people contributing to this group for political purposes are not disclosed. Groups with a 501c6 status with the IRS are generally business leagues that are also not required to disclose their donors.