Number of cloud platform (PaaS) offerings is increasing every day and it became quite a challenge to keep them all in mind, so I've started a spreadsheet that is updated by the cloud enthusiasts every day. As of May 2012 it contains a list of 70+ platforms with following information:

Name, Description, Language(s) supported, Frameworks Supported, Website, Development Status, Twitter, Vendor Name, Number of Developers, Number of available applications, Maturity (in years), Billing Model, Performance Metrics, Number of outages, Who is using it, User Rating (1-10), Comments.

The document also contains a second worksheet with other platforms, that go beyond generic application platforms - integration, security, middleware, mobility, engineering and so on. Feel free to add missing bits at this Google Spreadsheet