If you need something quick and dirty, you can just use PayPal's "Buy" buttons and drop them on your pages. These will take people off-site to PayPal where they can pay with a PayPal account or a credit card. This is free and super easy to implement.
If you want something a bit nicer where people pay on-site with their credit card, then you would want to look into one of those 3rd part payment providers. None of them (that I'm aware of) are completely free. All will have a per-transaction fee, and most will have a monthly fee as well.
Personally I've worked with Authorize.NET and PayPal Website Payments Pro. Both have great APIs and sample code that you can hook into via PHP easily enough.
