Custom error/success message in Magento

>> A simple and most needed thing that we frequently use in Magento that how to add custom Error and Success message in Controller page.

You just need to call these methods:
$session = Mage::getSingleton('core/session');  
// You can specify the session with yours like:  
// Mage::getSingleton('customer/session'),
// Mage::getSingleton('checkout/session')

$session->addSuccess('your message here');
$session->addError('your message here');