apex:selectList

<!-- Page: -->
<apex:page controller="sampleCon">
    <apex:form>
        <apex:selectList value="{!countries}" multiselect="true">
            <apex:selectOptions value="{!items}"/>
        </apex:selectList><p/>

        <apex:commandButton value="Test" action="{!test}" rerender="out" status="status"/>
    </apex:form>

    <apex:outputPanel id="out">
        <apex:actionstatus id="status" startText="testing...">
            <apex:facet name="stop">
                <apex:outputPanel>
                    <p>You have selected:</p>
                    <apex:dataList value="{!countries}" var="c">{!c}</apex:dataList>
                </apex:outputPanel>
            </apex:facet>
        </apex:actionstatus>
    </apex:outputPanel>
</apex:page>
            
/*** Controller: ***/
    public class sampleCon {
        String[] countries = new String[]{};
        //If multiselect is false, countries must be of type String
        //String countries;
            
        public PageReference test() {
            return null;
        }
            
        public List<SelectOption> getItems() {
            List<SelectOption> options = new List<SelectOption>();
            options.add(new SelectOption('US','US'));
            options.add(new SelectOption('CANADA','Canada'));
            options.add(new SelectOption('MEXICO','Mexico'));
            return options;
        }
            
        public String[] getCountries() {
            //If multiselect is false, countries must be of type String
            return countries;
        }
            
        public void setCountries(String[] countries) {
            //If multiselect is false, countries must be of type String
            this.countries = countries;
        }
    }