ReDJ is a native extension (component + plugin) available for Joomla that allow to manage URL redirections. This is very useful for creating URL aliases, and managing redirections from an old URL to a new one. A very similar extension has been added into Joomla since 1.6 version (Redirect component), but ReDJ supports powerful regular expressions that allow to match more than one source URL. In this way it is possible to create a group of redirections with just one ReDJ item. It's so easy.
Also, now ReDJ comes with support for macros into destination URL. This means that from now the destination URL can be dynamic.
Finally, an "internal redirect" feature has been added (Enterprise version only). With these features now is possible to create "real" URL alias with Joomla.
There are two versions of ReDJ, Community and Enterprise, both released under GPL license. The Community version is completely free, while the Enterprise version has a little subscription fee, but introduces some advanced features. Here's a comparison matrix between Community version and Enterprise version: