In December of 2009 I published a database of online giving marketplaces that was developed by a Coro Foundation Fellow working on behalf of the Hewlett Foundation. I'm pleased to provide this link to an updated database, also done by the Hewlett Foundation. You should download this document rather than try to read it on the screen. It now includes 100 such platforms.
Mashable just highlighted "11 Innovative Crowdfunding Sites for Social Good" - you can check out their list here.