Joomlatools

The legacy mode plugin that is provided with Joomla 1.5 is a small "layering" plugin that allows some of the old 1.0.x functions to be used in the shiny new Joomla 1.5 core. The legacy mode in effect allows you to take most if not all Joomla 1.0.x extensions and even some mambo ones and use them in your Joomla! 1.5 site without the need to await a 1.5 specific version.

By default this is disabled in Joomla 1.5 but it can be enabled quite easily. There is also some misconception that legacy mode is a bad thing to have on and it uses a whole mass of resources, there is a blog post here that goes over the ACTUAL memory usage of Legacy mode.