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Some of the entries are a bit out of date, as the list is only updated annually.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
Compilations of applications and resources on donation management and fundraising tools can be found at these two useful sites:
Tom Fraise also has a comparison matrix downloadable from his site, allowing you to compare applications according to a number of criteria.
Gunner and the Aspiration team have been an incredible inspiration and resource for us at OpenNews. Their guidance and feedback have shaped everything from how support relationship building amongst cohorts to planning goal-oriented meetings where participants leave with a feeling of accomplishment and a sense of purpose.
