Go to http://www.magentocommerce.com/magento-connect and type 'checkout' in the Search box.
You'll find there are dozens of extensions. Some are free.
But whichever one your choose, be aware that there's still a lot of filling out to do, albeit it on just the one page.
This often involves the use of clumsy drop-downs. And if PayPal is involved some checkout pages take you away from your site and you may lose your customer in the final step: payment!
The best plugin I know that makes the UX of any checkout page really smooth is Braintree's latest "drop-in" UI, known as "v.zero" and we do a Magento extension for it.