jQuery plugin for ajax select fill
Your demo throws errors in console, something about uniqueId in ui.selectmenu.

Anyway:


You can expose $.ajax deferred so someone may use all features of jQuery.Deferred which is already there.
Call success callback with data as argument.

settings.success.call(this, n);

If you do not want to do anything special in case of failure, and just want to provide this to error callback, you do not have to create closure there.

.fail( settings.error );


..., unless you want to hide other rejection arguments.
Do not use each if for do the same (for performance reason):

for( var index = 0; index &lt; n.length; index ++){
  list += '&lt;option value=' + n[index].Id + '&gt;' + n[index].Nazwa + '&lt;/option&gt;';
}

Personally I don't like single char variable names, as they are really painful for future development





Instead of $(this).empty();$(this).append(list); you can simply use $(this).html(list);
You can expose jQuery.Deffered, for example as this.promise = $.ajax(..., example in here http://jsfiddle.net/dhRRN/5/
I would suggest also applying received options to ajax call, so your plugin's user could also alter request itself.