I know what a singleton is, but while walking through a web-app, my co-worker said "singleton-modeless". What does he mean by this?
Well a modeless form is the opposite of a modal form.
modeless form
Allows you to change the focus from form to form.
modal form
Disallows focus change until the modal form is closed.
That being said, singleton modeless must mean a single instance of a modeless form.
I found a reference to it in the MSDN Architecture Center here. They appear to mean a window that is not modal (in other words, the user is not forced to interact with it before interacting with its parent) and where only one instance of it exists.
I'm not sure if that's what you're co-worker meant but I wouldn't feel bad about not knowing what he meant. He may not have actually known himself.
Singleton modeless or modal dialog is great because you don't have to worry that 2 instances will be shown to the user at the same time.
Ext.MessageBox is like this, called by a method, singleton modal. It could be used as the basis for a singleton modeless as well.
I agree, modal dialogs usually suck, unless very well thought through. And most modern apps are going modeless drawers, or ribbons, that are always available to the user while in context of the task at hand!
So examine your dialogs; you probably don't need half of them in your app right now...