Creating a Guest Sign In

I am interested in adding an additional sign in option for my customers when shopping on my website. I have noticed with some websites, for convenience sake, offer an option to sign in and pay for their items using a "sign in as a guest" option. I understand that this option may not allow for a user to create an account or establish a history or record of their purchases, but I have noticed that we have lost some customers because of the need to login before completing their purchase.

However, I still want to continue requiring a login option for those who have an account with us and desire to take advantage of some discount features we have incorporated into our eCart. In order to capture a larger percentage of customers wishing to shop on our website, I would also like to consider creating a guest sign option.

Do you know of any way this can be done; any online resources or sites that provide some detail in setting up a guest option? Do you know of any existing forum thread you have addressed that has outlined the creation of such an option?

Any assistance you may offer would be greatly appreciated.