In Magento, If you are on category page than it's easy to get current category.
$_current_category=$this->getCurrentCategory();
or,
$_current_category = Mage::registry('current_category');
I needed Current Category on product info page and on the footer.phtml files. Because I had custom background set for each Category. So, Below code helped me to find the Current Category in my Magento's phtml file.