You get what you pay for

In some instances, content providers require you to accept a license agreement in which you promise to display the "free" content in its entirety - that means you can't modify the content at all. This is usually not a big deal but sometimes it is because you may end up with content that is not so great.

But you said it was Free

Yeah sure, content is free but only for academic, personal or nonprofit use.

If your site is a commercial one in the sense that it's for profit purposes - such as displaying paid advertising and/or selling products (books come to mind), you may be required to pay for the content you want to display on your site. In some cases, all you may be required to do is contact the content provider (in most cases the webmaster) and get permission to use it's content.

A word of advice

Before you sign up with free content providers, always read their Terms of Service or License agreements carefully and make sure that you read the fine print.

Asking people - who already signed up with such services - about what they think about their content provider may also help you find/select the right content provider for you.

Now, let's go ahead and take a look at a few content providers.