If you don't have a merchant account or don't have the budget to get one, there are several inexpensive alternatives you can use to accept payment from your website.

PayPal enables anyone with an email address to send, receive and accept online payments on their website. There are no monthly fees, but they do charge a very low percentage of the transaction value. PayPal is great if you sell digital products online (ebooks, software) to provide your clients with instant access. It's free to sign up. Click here to read an article about Paypal.

Here are a few more companies that offer this similar service:

2Checkout.com
Clickbank.com
Princess Card
E-Gold
StormPay
Hyperwallet