Amazon.com current has Buy 4 Items for the Price of 3 Promotion with many eligible products.  Essentially, the way it works is that if you have 4 eligible items in the cart, an automatic discount will be applied to the lowest-priced item during checkout to make it free.  Note that all items have to be sold by Amazon.com itself.  At the time of this post, there are more than 45,000 items eligible under this promtion.  Just for kicks, we have a screenshot here that shows a saving $7900!
Originally posted earlier, this is just a friendly reminder.