Mini Script Editor is a development version of an AppleScript editor which compiles live, as you type.
It compiles the script live, as you type - no need to explicitly compile the script (though can do that too, by pressing the enter key). It also shows errors in a status bar, silently (it would be a real pain to have live compilation but show errors in alert dialogs).

It cant save scripts yet. You can load existing scripts, or type your own, but you cant save them yet. This is very much an early work-in-progress application.