paypal.com, moneybookers.com, plumus.com, 2checkout.com, e-gold.com, epassporte.com, e-bullion.com etc.etc.... but out of all paypal is on top in the market...
If you are in the United States, you have hundreds of processors to choose from and scores of electronic payment gateways. Most of these gateways (Quantum, LinkPoint / YourPay, Authorize.net / Cybersource, Payflow, Yahoo!®, etc) that will connect to a transaction processor (First Data, Nova, etc). Some of the transaction processors that have a relationship with the issuing bank and acquiring bank can complete the transaction instead of sending it to the card associations (Visa, MasterCard, American Express, Discover, etc). You might even remember the lawsuit that Visa was suing First Data because Visa was getting cut out of a lot of the transactions because First Data was completing the transaction. Fortunately, that was settled. The transaction can be broken down into about seven steps Authorization Merchant Balancing Capture Capture Interchange (V/MC Only) Interchange (V/MC Only) Merchant ACH The electronic payment gateway will virtually connect your website to a transaction processor. If you are in the United States, you have a number of options. Getting a merchant account is pretty good if you are doing more than $1,000 a month. One of the bigger reasons is getting your money. Some think this is not important but I have to object. Getting your money is usually the best thing, right? And with a merchant account, your money is deposited into your bank account within 48-72 hours usually after the batch is completed. If you do the math and look at what you might earn if the money if in your interest bearing account you might make money with your money. The IPSP (Internet Payment Service Provider) like (some versions of) PayPal are relying on your money to earn interest. Some of them will send your money weekly or twice a month, while some will send it only when you request it. The electronic payment gateways will also offer you an API to process the transaction on your website. This helps with the flow of the checkout process. The user is never directed to another website. You can get a test store from most of the electronic payment gateways but some of the skilled developers think this is unnecessary. And others think it is a great option to have. Now, even some members of Paypal can get this as well. Now, if you are in the United States, there are some very important pieces of information must be on an ISO / MSP / Agent's website. Let's take a look at CDG Commerce's web site and look at their footer: This tells you CDG is a registered ISO/MSP and what bank he is with. An agent will have something like "independent agent office" with a certain ISO. If you are a member of Sam's Club or Costco, you might check their programs to see if they are suitable. Just make sure to read the fine print and all the legalease. The electronic payment gateways might charge you a transaction fee - some based on a fixed fee and some a percentage. Some gateways will give you XXXX number of transactions free per month and then charge you. And some gateways will just charge a flat fee (similar to AOL giving away their software so you will use service). It is the gateway that is really important. When the gateway is down - your ecommerce site is basically shut down. Although some gateways offer a way for you to still send the transaction and once the gateway is back up, the transaction will be processed. But I am guessing that the question might need a bit more information
Worldpay, chronopay.. but as the others say, the more info you give about your self the easier it is to give you a proper suggestion