Fyodor is an open source Java library for generating random data. It's distributed under the MIT license and hosted on GitHub.
It's available in modules, the core module is dependency-free and it's an important goal for us that it stays that way. Currently there is also a module supporting Joda Time, and a JUnit-dependent module that allows you to reproduce tests that have failed with some random data.
Read more about why random data generation in your tests is a good idea and how to get the best from it in Random Data and Builders or dive into the User Guide to get started.