Ryan has discontinued support and development for his original Advanced Tags and Prefixes application, and it hasn't seen much further development since its initial launch anyways as Ryan hasn't had the time to work on it.
Radical Tags has been rewritten from the ground up for the latest releases of IPS.
It replaces the old prefix editor which requires HTML knowledge and manual styling with an easy to use editor anyone can use, or, for more advanced users, provides a fully featured templating system.
In addition, it adds support for tag aliases, automatically converting tags to prefixes, more advanced tag management features, and in the future will support things like multimoderation tools for tags and Prefixes, prefixes for gallery albums, tag/prefix filtering and more.