Tinypass is a web service that makes it easy charging your users for content.
The service is really easy to integrate and allows to start charging for content on the web.
The idea usually is such that you make users pay for promoted content (if you own a paper magazine or something akin to it). The payments are done in-page so the user never really leaves the content.
You could, by the way, sell HTML e-books, etc., with a service such as this.