I found this terrific free Million Dollar Donor List from The Center for Philanthropy at Indiana University. The Center converted the subscription-only list to a free online resource in May 2011.
According to the website, the current list is a record of publicly announced charitable gifts of $1 million or more from 2000 to 2010 given by U.S. residents to domestic or international entities across a range of charitable subsectors. The list is updated on a quarterly basis. It contains only publicly announced gifts.
The website allows a person to download the entire dataset, which could be very useful when looking for particular types of principal gift donors.
Enjoy!