For basic financial transactions, nothing beats PayPal for quick-and-easy. Whether you want to collect donations, sell stuff, or gather subscription payments, PayPal has taken the pain out of getting paid. See http://www.paypal.com for the scoop.
If you're ready to launch full force into putting up stuff for sale on your site, you want to get informed about the ins and outs of e-commerce. A professional Web designer we know recommends this site for information minus the hype: http://www.practicalecommerce.com.