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If you use PayPal but the site you want to buy from does not, use this browser plugin (ie7 and FF2 in Windows).

Lets you pay with your preferred method by generating card numbers to satisfy the website. I dig it, but alas I must wait until its compatible with FF3 because I am too lazy to use something else. Paypal had introduced the method of using them anyway awhile back. Now it comes in the convenient form of a plugin.