According to this Consumer Reports article, Free Shipping Day has over 700 participants offering (at a minimum) free standard shipping on orders delivered in time for Christmas.
Participating stores are encouraged to offer blanket free shipping whenever possible, and discouraged from pulling the old "except for items in these 1500 categories" trick.
Free shipping offers are available year round, of course. Amazon.com has one of the more famous free shipping offers with their Super Saver shipping program, and other merchants like Newegg and Buy.com offer a ton of items with free shipping.
Free Shipping Day is an opportunity for many more stores to fight over your online shopping dollars, however. By providing free shipping coupon codes in the waning days of the 2009 holiday shopping season, you may be able to obtain a deal you couldn't at other times of the year.
Maybe this is the time to pull the trigger on that giant television, or refrigerator, or grand piano full of bowling balls you've been looking for.