The Open Scripting Architecture (OSA) is Apple’s AppleEvents based mechanism for controlling an application’s behavior through whatever OSA compatible scripting language the user desires. At this time, the most popular (to the point of near exclusivitity) OSA-compatible scripting language is Apple’s own AppleScript.

Which actually isn’t very popular among developers (from what I gather) but is really the only choice.

Well, Python has good OSA components (appscript rocks!) and JavaScript has a nice OSA implementation… – l0ne aka EmanueleVulcano