aheadWorks -
One Step Checkout
The extension reduces the number of checkout steps from 6 to 1 and allows customers to buy products in one simple click.

Features for customers:

Checkout on a single page
Login/registration option on the checkout page
Ability to save address in address book
Auto-fill billing and shipping addresses for logged in customers
Send to different addresses
AJAX-based update of each step
Gift options support
Apply discounts
Leave a comment
Subscribe to newsletter
Remove products
Sage Pay is supported
iPad/Tablet compatibility

Features for online Magento store owners:

Control the content of the checkout page easily
Specify Magento checkout page title and description
Default shipping and payment methods
Blocks numbering on the checkout page
Display Terms & Conditions checkbox
Cross-sells appearing

Compatibility with the following Magento extensions by aheadWorks:

AJAX Cart Pro
Facebook Link
Gift Wrap
Points & Rewards
Refer a Friend v.2.x
Delivery Date and Notice
Advanced Newsletter
Automatic Related Products 2