The Open Scripting Architecture (OSA) of MacOs and MacOsx allows developers to implement new AppleEvents-capable scripting languages by writing a ScriptingComponent, of which AppleScript is an example.  Although the OSA is not required for a script to send AppleEvents, using it allows applications which load, compile, or execute scripts to work with scripts in that language.

In addition to driving applications via AppleEvents, OSA scripts can also make use of ScriptingAddition''''''s.

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What languages support OSA, and where can I find info on how to use it with each of them?

I hear JavaScript has support, but I know not where.

http://www.applescriptsourcebook.com/links/languages.html seems to have more info (although a lot of it sounds out of date).