Frequently Used Methods in Magento

1. Check if customer is logged in:

if(!Mage::getSingleton('customer/session')->isLoggedIn())
{
 $this->_redirect('customer/account/login/');
}

2. Get current site url:

<?php Mage::getUrl(); ?>

3. Get Category URL:

$categoryUrl = Mage::getModel('catalog/category')->load($categoryId)->getUrl(); 

4. Get product URL:

$productId = 1;
$product = Mage::getModel('catalog/product')->load($productId);
$path = Mage::getUrl().$product->getUrlPath();

5. Get Product image path:

$productId = 1;
$product = Mage::getModel('catalog/product')->load($productId);
$path = Mage::helper('catalog/image')->init($product, 'image')->resize(75, 75);

6. Format a number as price:

<?php $this->formatPrice(10);?>

7. Call a .phtml explicitly:

echo $this->getLayout()->createBlock('sales/order_recent')->setTemplate('sales/order/recent.phtml')->toHtml();

8. Get Currency Code:

Mage::app()->getStore()->getCurrentCurrencyCode();

9. Get Currency Symbol:

Mage::app()->getLocale()->currency(Mage::app()->getStore()->getCurrentCurrencyCode())->getSymbol();

10. Get Customer Shipping/Billing Address:

$customerAddressId = Mage::getSingleton('customer/session')->getCustomer()->getDefaultShipping();
if ($customerAddressId){
       $address = Mage::getModel('customer/address')->load($customerAddressId);
}

11. Get products details in the cart:

$mageFilename = 'app/Mage.php';
require_once $mageFilename;
umask(0);
Mage::app();
 
// Secret Sauce - Initializes the Session for the FRONTEND
// Magento uses different sessions for 'frontend' and 'adminhtml'
Mage::getSingleton('core/session', array('name'=>'frontend'));
 
$session = Mage::getSingleton('checkout/session');
foreach ($session->getQuote()->getAllItems() as $item) 
{
 $productid      = $item->getProductId();
 $name       = $item->getName();
 $qty       = $item->getQty();
  
 echo "Product Id-".$productid.', Name-'.$name. ', Quantity-'.$qty;
}

12. Find which class is loaded in current template:

<?php
    print_r(get_class_methods(get_class($this)));
?>
or
<?php
    print_r($this->debug());
?>
or
<?php
    echo Zend_Debug::dump($this);
?>

13. Get Current theme folder:

<?php echo $this->getSkinUrl('') ?>

14. Get total price of items in the cart:

<?php echo $this->helper('checkout')->formatPrice(Mage::getSingleton('checkout/cart')->getQuote()->getGrandTotal()); ?>

15. Get the current category:

<?php
    $currentCategory = Mage::registry('current_category');
?>

16. Add product to cart through querystring:

http://www.mydomain.com/checkout/cart/add?product=[id]&qty=[qty]
 
http://www.mydomain.com/checkout/cart/add?product=[id]&qty=[qty]&options[id]=[value]

These are the some of the frequently used functions in Magento

Hope it Helps... Thanks..