From the Hibernate Related Projects page, Hibernator is an Eclipse plugin for Hibernate.
Download and install the plugin and it will automatically generate hbm mapping files from existing Java source files (open up the source in question, then go to Window –> Show View –> Other –> Hibernator) and can also be configured so that you can run run Hibernate queries in real time (I couldn’t get that feature to work, it won’t find my persisted classes).