PayPal is offering a 20% back promotion on total purchase amounts (including shipping and taxes) made with participating merchants.  The maximum total amount of cash back per PayPal account is $50 USD and the offer runs until January 10, 2008.
 
You can find participating merchants here:  https://www.paypal-promo.com/holiday/slider/index.html?s=cashback  (Once you choose your category, just use your mouse to pull the pointer on the slider to view all merchants.)
 
Be sure to read the Terms and Restrictions here:  https://www.paypal-promo.com/holiday/terms_slider.html