Political gifts of at least $200 made by individuals to federal candidates or political action committees must be reported to the public by the Federal Election Committee. Drill down to see who else has given to the same recipient. Additionally, it does a number of things for you: it groups contributions under aliases used by the donor and by recipients and totals donations of the same kind. Includes an excellent advanced search feature that allows you to limit your search by city, state or zip.
The NY Times has put together an excellent guide to political donations, what must be disclosed, what the limits are, and how big money donors can donate without disclosing their identities.