Light weight alternative to Hibernate?

Hibernate requires next to zero configuration if you use annotations. It even can auto-discover mapped beans in the classpath! I don't see any alternative to it from both simplicity and power point of view.

It can also expose itself as JPA, which is (IMHO) even simpler.

It still requires XML configuration, but have a look at MyBatis (formerly iBatis).

My ORMLite library is one such alternative. It supports MySQL, Postgres, Microsoft SQL Server, H2, Derby, HSQLDB, and Sqlite, and can be easily extended to others. It uses annotations to configure classes, good Spring support, flexible query builder, etc..