For developers, one of the many cool things about using a PaaS is that you can easily connect your application to any type of service instead of being limited to what your IT department officially provides. That means you can use the best tool for the job, or even that you can finally find an excuse to play with a technology you hadn't previously been able to.
With a purely hosted PaaS, such as Heroku, your choices are limited to the services that Heroku offers, whether natively or through the Heroku Add-Ons marketplace. With a private PaaS, such as Stackato (based on Cloud Foundry) or Cloud Foundry itself, you have your choice of the provided services but can easily add almost anything.
Stackato comes with the following data services up and running, out of the box: MySQL, PostgreSQL, Redis, MongoDB, Memcached, and Stackato filesystem. For completeness, Stackato also includes a log service (Logyard), messaging service (RabbitMQ), and port service (Harbor).
Other available options include a variety of services that have been open-sourced in the past, including CouchDB, ElasticSearch, and ActiveMQ. (Note that all three of these options were written as Cloud Foundry v1 compatible services.) There are also fully supported add-ons such as Stackato's Oracle 11g service connector (connects Stackato to your Oracle service).
If those ActiveState-backed options aren't enough, there is another world of Stackato-compatible services available from the Cloud Foundry community, including SAP HANA (contributed to the Cloud Foundry incubator by SAP). Aside from HANA being really interesting in the enterprise world, it's worth mentioning that SAP is, like ActiveState, a founding member of the forthcoming Cloud Foundry foundation; you can be assured SAP is committed to ensuring HANA is a first-class citizen of Cloud Foundry.
If you, like seemingly everybody else in tech, are captivated by Docker, then the new, experimental, Docker Service Broker for Cloud Foundry might be right up your alley. As its name indicates, it lets you use a Docker-based service with Cloud Foundry (or Stackato!) and seems like something that would be fun to play with. (Fun fact: Stackato already uses Docker for its application containers.)
If you want to write your own service broker, an excellent way to get started is with our open-source Node.js Service Broker. (That blog post also contains some useful background on the differences between the Cloud Foundry v1 and v2 services model.)
The options above require admin or "operator" level access to Stackato and/or Cloud Foundry. However, even if you're just an end-user of either system, you can still easily specify your own services with the little known feature called user provided service instances. (The author of that post often walks around our office proselytizing this as the greatest new feature of last year's Cloud Foundry v2 rewrite; he will no doubt appreciate this link.)
Did I miss anything useful, cool, or interesting? If so, let me know in the comments.