Listing all "SalesForce Objects (Standard/Custom)" to VF page

Please try below code for custom object with custom field

public with sharing class ExtractSobject
{
    public String Sf{get;set;}
    public String Selectedobject { get; set; }    
        public List<SelectOption> getSelectedobjnames()
        {
            List<Schema.SObjectType> obj = Schema.getGlobalDescribe().Values();
            List<SelectOption> options = new List<SelectOption>();
            options.add(new SelectOption('--Select Object--','--Select Object--'));
            for(Schema.SObjectType st : obj)
            {
                if(Selectedobject == null || Selectedobject=='' )
                {       
                    Selectedobject = st.getDescribe().getName();
                }
                options.add(new SelectOption(st.getDescribe().getName(),st.getDescribe().getName()));
            }
            return options;
        }
        public List<SelectOption> getSelectedobjFields()
        {
            SObjectType objTyp = Schema.getGlobalDescribe().get(Selectedobject);
            DescribeSObjectResult objDef = objTyp.getDescribe();
            Map<String, SObjectField> fields = objDef.fields.getMap();
            Set<String> fieldSet = fields.keySet();
            List<SelectOption> options = new List<SelectOption>();
            options.add(new SelectOption('--Select Object--','--Select Object--'));
            for(String s:fieldSet)
            {  
                SObjectField Sobjfields = fields.get(s);
                DescribeFieldResult selectedField = Sobjfields.getDescribe();                 
                options.add(new SelectOption(selectedField.getName(),selectedField.getName()));
            }
            return options;
        }
}

<apex:page controller="ExtractSobject">
<apex:form >
   <apex:pageblock >
     <apex:pageblocksection >
       <apex:pageBlockSectionItem >
         <apex:outputlabel value="Select Object"/>
          <apex:selectList value="{!Selectedobject }" size="1">
            <apex:selectoptions value="{!Selectedobjnames}"></apex:selectoptions>
            <apex:actionSupport event="onchange" rerender="a"/>
         </apex:selectList>
       </apex:pageBlockSectionItem>
     <apex:pageBlockSectionItem >
      <apex:outputPanel id="a">
         <apex:outputLabel value="Object Fields" ></apex:outputLabel>
             <apex:selectList value="{!Sf}" size="1"> 
                   <apex:selectOptions value="{!SelectedobjFields}" />
             </apex:selectList>
      </apex:outputPanel>
     </apex:pageBlockSectionItem>
   </apex:pageBlockSection>   
  </apex:pageblock>
 </apex:form>
</apex:page>