I wrote this extension for a series of posts where I discussed Magento checkout customization. This extension adds a OneStep checkout to Magento. A OneStep checkout differs from the standard Magento OnePage checkout by arranging every checkout stage (billing and shipping addresses, shipping details, payment information, and order review) in one step – hence the name.
Screenshot: OneStep Checkout, non-virtual product
Screenshot: OneStep Checkout, virtual product.
You can get this extension from GitHub: Divisionlab oneStep Checkout for Magento.