If you don't care about the customer leaving your site:
Amazon Payments (underrated in my opinion)
PayPal
Google Checkout
If you need to seamlessly integrate a payment form in your site:
-PayPal Payments Pro ($30/month for a virtual terminal and API integration, no contracts, 2.9% transaction, it's really hard to find a merchant company that can beat this unless you're talking in volumes).
-Authorize.net coupled with the merchant company of your choice.
When evaluating a merchant company, beware of contract stipulations, transaction fees, and monthly fees. My first provider was Cardservice International (one of the largest) and I used LinkPoint for the online gateway. I was paying $60-80/month just for the service! (transaction fees excluded) AND I was locked in a 2-year contract with an insanely high cancellation fee. One of their fees was a mandatory $20 "customer service fee" for access to their 24/7 customer service team. (WTF?)
Just be careful when evaluating merchant companies. Try starting at your bank (as long as it's not Chase, BoA, or Wells Fargo). Generally, local community banks or credit unions will have fair pricing and good customer service for merchants.Try to check all
Honest Merchant Report here.