1. Salesforce's Force.com

2. Amazon's EC2

3. Google's AppEngine

4. Microsoft's Windows Azure

5. VMWare/Salesforce's VMForce

6. Makara

7. CloudBees

8. PiCloud

9. PHPFog

10. DuoStack

I'm not sure it's obvious that PaaS is "winner take all", but even if it is, right now there are several deep-pocketed companies fighting to be that winner. Sounds pretty good if you're looking for an exit.