1. Heroku Add-ons:

https://addons.heroku.com/

2. EngineYard Cloud Products Add-ons:

http://www.engineyard.com/partners/platform-services

3. MS Azure Store:

http://www.windowsazure.com/en-us/store/overview/

4. Amazon AWS Marketplace (not really PaaS, but close):

https://aws.amazon.com/marketplace

Any others?