How to make a dynamic picklist in salesforce?

Hello,

A dynamic picklist allows end users to add values to the list rather than requiring the assistance of an administrator. It can be updated as and when values are required. This is particularly useful if you have processes within your organization whereby you would have to request values to be added to a standard picklist. This could take some time and if you're in the middle of something, it would distract from what you're doing and delay things unnecessarily.

Thanks.

Apex class
public class ScemaMultiDisplayEx {
public list<selectOption>notSelectedOption{set;get;}
public list<selectOption>SelectedOption{set;get;}
public list<selectOption>SelecteObject {set;get;}
public set<string>NotSelectedFields{set;get;}
public set<string>SelectedFields{set;get;}
public list<string>FieldNames{set;get;}
Public list<string>nonSelected{set;get;}
Public list<string>Selected{set;get;}
public map<string,schema.SObjectType>ObjectMap    {set;get;}
public map<string,schema.SObjectField>fieldMap {set;get;}
public list<Sobject>result                     {set;get;}
public string SelectedObject                   {set;get;}
public ScemaMultiDisplayEx(){
ObjectMap= new map<string,schema.SObjectType>();
notSelectedOption= new list<selectOption>();
SelectedOption= new list<selectOption>();
SelecteObject=new list<selectOption>();
NotSelectedFields= new set<string>();
SelectedFields = new set<string>();
Selected= new list<string>();
nonSelected= new list<string>();
ObjectMap=schema.getGlobalDescribe();
fieldMap=new map<string,schema.SObjectField>();
selectOption op = new selectOption('None','-None-');
SelecteObject.add(op);
notSelectedOption.add(op);
SelectedOption.add(op);
for(string s:ObjectMap.keySet()){
selectOption op1= new selectOption(s,s);
SelecteObject.add(op1);
}
}
public void getOption(){
schema.SObjectType obj = ObjectMap.get(SelectedObject);
schema.DescribeSObjectResult res=obj.getDescribe();
FieldMap=res.Fields.getMap();
NotSelectedFields.addAll(fieldMap.keyset());
getValue();
//system.debug(NotSelectedFields);
}
public void getValue(){
notSelectedOption.clear();
SelectedOption.clear();
Selectoption op= new selectoption('None','-None-');
if(NotSelectedFields.size()>0){
for(string ss: NotSelectedFields){
selectoption p1= new selectoption(ss,ss);
notSelectedOption.add(p1);
}
}else{
notSelectedOption.add(op);
}
if(SelectedFields.size()>0){
for(string s:SelectedFields){
selectoption p= new selectoption(s,s);
SelectedOption.add(p);
}
}else{
SelectedOption.add(op);
}
}
public void AddRecords(){
NotSelectedFields.removeAll(Selected);
SelectedFields.addAll(Selected);
getValue();
}
public void RemoveRecord(){
SelectedFields.removeAll(nonSelected);
NotSelectedFields.addAll(nonSelected);
getValue();
}
}
Visulforce page
<apex:page controller="ScemaMultiDisplayEx" showHeader="false" >
<style>
.b{
width:200px;
height:200px;
color:Blue;
}
</style>
<apex:form >
<apex:pageBlock >
Choose a Object <apex:selectList size="1" value="{!SelectedObject}">
<apex:selectOptions value="{!SelecteObject}"/>
<apex:actionSupport event="onchange" action="{!getOption}" reRender="one" />
</apex:selectList>
<apex:panelGrid columns="3" id="one">
<apex:selectList multiselect="true" value="{!Selected}" styleClass="b">
<apex:selectOptions value="{!notSelectedOption}"/>
</apex:selectList>
<apex:panelGrid columns="1">
<apex:commandButton value="ADD" action="{!AddRecords}" reRender="one" />
<apex:commandButton value="DEL" action="{!RemoveRecord}" reRender="one" />
</apex:panelGrid>
<apex:selectList multiselect="true" value="{!nonSelected}" styleClass="b"  >
<apex:selectOptions value="{!soption}"/>
</apex:selectList>
</apex:panelGrid>
</apex:pageBlock>
<apex:pageBlock title="Records" rendered="{! !Isnull(result)}">
<apex:pageBlockTable value="{!result}" var="a">
<apex:repeat value="{!FieldNames}" var="b">
<apex:column value="{!a[b]}" />
</apex:repeat>
</apex:pageBlockTable>
</apex:pageBlock>
</apex:form>
</apex:page>