I am a developer. How can I make my applications AppleScriptable?

Apple has made available documentation to aid you in adding AppleScript support to your own custom applications. This documentation can be found here.

In addition, the MoreOSL C library helps carbon developers to implement scriptability in their applications.

You can allways find lots of help at Apple’s applescript-implementors mailing list and its archives.

Extra tools, source code and reference material here:
Don Brigg�s Suite Modeler and sample scriptable cocoa applications.
The GraphicKit, by John HÃ?¶rnkvist, a powerful sample scriptable framework.
Andrew Stone (author of Stone Studio), also wrote an article about ââ?¬Å?Adding AppleScript Support to Cocoa Appsââ?¬ at MacTech.