This answer is framework-specific, but I'm working on adding support for this tool in doctrine/common, so if you are using symfony or CI or other frameworks, this may also be available soon.
If you are using Zend Framework 2.1 (as I'm writing) and follow my tutorial on installing Doctrine 2 ORM with ZF2, the developer toolbar will give you the ability to generate a diagram of all the entities with their relations, as of following picture:
The tool is not yet perfect, but you may find it and help improve it in DoctrineORMModule.
