Vision Factory is a multiplatform software (MacOSX / Windows) written in C++ that seems like a promising alternative to processing or Max/MSP. It is aimed at creating real-time interactive visual animations by the programming of reusable and sharable plug-ins. Through its modular architecture, one can quickly build complex scenes by using data resources (images, webcam feed, sounds, osc/midi messages, web services, etc) through the use of dedicated built-in libraries.
Vision Factory relies on several third-party libraries and has numerous facets making it a rich framework for the experimentation and the creation of visuals. It is well suited for installation and/or vj shows for example and was primarily thought for easily handling a multi-computers rendering system by providing native network capabilities.
The beta version of the software is being released in May 2007 but you can leave your e-mail in the contact form in order to be notified.
You can read more about Vision Factory here.