StreamByter is a powerful scripting language for writing MIDI processing scripts from Audeonic. There is a standalone app called MidiFire that lets you create and use StreamByter scripts as well as an AU that can host them. MIDI Designer Pro can also make use of StreamByter scripts.
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The developer is quite active on a forum devoted to MidiFire and StreamByter and is very helpful in helping develop scripts. The forums is found at: http://audeonic.boards.net