Well, I just did a little bit of searching for PayPal alternatives, and here is what I found:

Verisign
Cardservice International
Miva Merchant

Not sure what all the benefits / downfalls of each are, but you can check them out.

I think those are all companies that you can perform authorizations through directly. More than likely, that is not what you want to do. Performing authorizations yourself is not a fun thing. You have to be EXTREMELY careful on how you design things, make sure there are no loopholes or security breaches (thats about 1000 times harder than it seems), make sure all data gets recorded properly, nobody gets billed more than once, everybody that gets billed successfully makes it through to completion....a lot of trouble if you want to do it right. If you dont do it right, you leave yourself open to massive liability.

Thats why most people use a 3rd party credit card processor. They are responsible for all of the above and just take a small fee. Unless you are doing massive sales (hundreds of thousands of dollars) you are likely to find 3rd party processors to be a better deal. Even then it may still be a bargain to save you from the headache of doing it yourself.

As mentioned in the FAQ, look here: http://www.blackcatsystems.com/regservices/

www.paypalwarning.com

www.2checkout.com

It seems pretty good to me...