OpenJPA, obviously. I've also seen EclipseLink and Hibernate mentioned, but is there significant difference in capabilities?
Most differences come from how OSGi-aware the provider is. For example, you may need to wrap the Hibernate jars as OSGi bundles yourself (or use a set someone else has already wrapped - there are a few floating around on the web).
Occasionally providers try and do classloading in such a way that it won't work in an OSGi environment, so this is something to look out for if you wrap your own or use 'off-piste' features of the main supported providers.
You'll also need to make sure the provider registers itself as an OSGi service. This just works with OpenJPA, and there's an EclipseLink adapter in the Aries codebase. I assume the Hibernate bundles people have shared do this as well. There are instructions on how to do this for other providers (DataNucleus in particular) in this thread:
http://www.mail-archive.com/aries-user@incubator.apache.org/msg00251.html