I need to run a very simple mac application of a few kb, is there an emulator of some really old mac os that can be run in a window like an application?

None exist. If one does, it certainly isn't legal in any respect, at least.

The only thing that exists that may some day be capable is OpenDarwin/PureDarwin, but that is in it's infancy and not compatible with anything earlier than Mac OS X.

What's the application? Surely there could be an open source alternative to it.

Emulators are not a topic we want discussed here.