A simple 10 mins jquery based expandable countries selector. Ex: http://jsfiddle.net/leonel0101/UyfxM/6/

ExpandableCountriesSelector

 $(function () {
     var makeExpandableCountriesSelector = function (selector, def, aditional) {
         // LOCAL CONSTANTS
         var LESS_VALUE = 'less',
             MORE_VALUE = 'more';
         
         // Our model
         var countries = def,
             more = true,
             less = false;
        
         // Utility method to render an option simple html
         var renderOption = function (value, text) {
             return '<option value="' + value + '">' + text + '</option>';
         };
         
         // Updates the select element
         var updateCountries = function () {
             // We render the select country text.
             var rendered = renderOption('void', 'Select Country...'); 
             
             // We reduce our model and render each option
             rendered += countries.reduce(function (prev, curr, idx) {
                 if (idx == 1) prev = '<option value="' + prev + '">' + prev + '</option>';
                 return prev + renderOption(curr, curr);
             });
             
             if (more) rendered += renderOption(MORE_VALUE, 'Show More...');
             if (less) rendered += renderOption(LESS_VALUE, 'Show Less...');
             
             $(selector).html(rendered);
         };
         
         
         // Event delegation
         $(selector).change(function () {
             if ($(this).children(":selected").val() === MORE_VALUE) {
                 // Change the model
                 countries = def.concat(aditional);
                 more = false;
                 less = true;
                 // Update the View
                 updateCountries();
             }
             if ($(this).children(":selected").val() === LESS_VALUE) {
                 // Change the Model
                 countries = def;
                 more = true;
                 less = false;
                 // Update the View
                 updateCountries();
             }
         });
        
         // Start!
        updateCountries(true);
     };
     
     // Usage: 
     // makeExpandableCountriesSelector("select#test", 'Guatemala,El Salvador, Honduras'.split(','), 'Uruguay,Chile,Panama,Brazil,Argentina'.split(','));

 });