Here is a good list of online donation processors: http://www.affinityresources.com/pgs/awz55Online2.shtml
Some of the entries are a bit out of date, as the list is only updated annually. Most entries are correct, though. Note that there is a 2nd page with additional information.
(Thanks, Robert Weiner!)
Addendum: There is also a useful comparison chart of online registration ASPs on the Consultant Commons at http://www.consultantcommons.org/node/37