PayPal browser plugin: Buy stuff on sites that don't take PayPal
You can install the PayPal plugin by logging into your PayPal account and select PayPal Plug-in from the menu on the left. You'll be prompted to download a 5MB file which will install the plugin for both Internet Explorer and Firefox. Note that the plugin is Windows only, and does not seem to play well with Firefox 3 yet.
Once installed, a window should pop up whenever you visit a site asking for your credit card information. You can also generate a card manually through a drop down menu.
If you're using Mac, Linux, or Firefox 3, you can still use the secure card feature by logging into your PayPal account and generating a card number. But the browser plugin makes the process much easier.