Heroku, AWS, GAE are popular and well-known... but by tightly integrating an application or service to a proprietary platform, you're pretty much at the mercy of the PAAS provider.

To avoid vendor lock-in, check out OpenShift[1] from Red Hat. It's open source[2] so you can set it up on your own servers if you felt Red Hat weren't providing the service you required, or as redundant servers for fail-over.

[1] https://openshift.redhat.com/community/paas

[2] https://openshift.redhat.com/community/open-source/download-...