Create Custom Tab under Sales->order in Magento Admin

Create: app/code/local/Itflex/CustomSalestab/etc/config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Itflex_CustomSalestab>
            <version>1.0.1</version>
        </Itflex_CustomSalestab>
    </modules>
    <global>
        <blocks>
            <customsalestab>
                <class>Itflex_CustomSalestab_Block</class>
            </customsalestab>
        </blocks>
    </global>      
    <adminhtml>
       <layout>
            <updates>
                <customsalestab>
                    <file>itflex_customsalestab.xml</file>
                </customsalestab>
            </updates>
        </layout>
    </adminhtml>
</config>

Create: app/code/local/Itflex/CustomSalestab/Block/Adminhtml/Order/View/Tab/Customtab.php

<?php
class Itflex_CustomSalestab_Block_Adminhtml_Order_View_Tab_Customtab
    extends Mage_Adminhtml_Block_Template
    implements Mage_Adminhtml_Block_Widget_Tab_Interface
{   
    public function _constuct()
    {
        parent::_construct();
        $this->setTemplate('itflex/customsalestab/order/view/tab/customtab.phtml');
    }

    public function getTabLabel() {
        return $this->__('Custom Tab');
    }

    public function getTabTitle() {
        return $this->__('Custom Tab');
    }

    public function canShowTab() {
        return true;
    }

    public function isHidden() {
        return false;
    }

    public function getOrder(){
        return Mage::registry('current_order');
    }
}
?>

Create: app/design/adminhtml/default/default/template/itflex/customsalestab/order/view/tab/customtab.phtml

<?php echo 'add your html/php code here'; ?>

Create: app/design/adminhtml/default/default/layout/itflex_customsalestab.xml

<?xml version="1.0"?>
<layout>
    <adminhtml_sales_order_view>
        <reference name="sales_order_tabs">
            <action method="addTab">
                <name>order_view_tab_order</name>
                 <block>customsalestab/adminhtml_order_view_tab_customtab</block>
            </action>
        </reference>
</adminhtml_sales_order_view>
</layout>
Create: app/etc/modules/Itflex_CustomSalestab.xml

<?xml version="1.0"?>
<config>
     <modules>
        <Itflex_CustomSalestab>
            <active>true</active>
            <codePool>local</codePool>
        </Itflex_CustomSalestab>
    </modules>
</config>