Many handmade sellers are skeptical of free shipping, because free shipping isn't free for the seller. This attitude though ignores the psychological aspects of how you set your prices. Here are the two most common scenarios for free shipping:
Always Free - The seller offers free shipping all the time. In this scenario the seller has to build the cost of shipping into the price of the items.
Free as Sale - The seller occasionally offers free shipping as part of a sale or promotion. In this scenario the seller needs to have a high enough profit margin all the time to afford to run discounts*
However why offer free shipping when you still have to pay for it? For the same reason that a seller would offer 10% offer or run advertising: as a promotional tool. "Free" is a very powerful word and for online retailers an easy and affordable way to bring in extra business. In fact at least one consumer researcher has shown that shoppers will often pick free shipping over a discount even if the discount would save them more money.
For the time being I'm only offering free shipping as an occasional discount but with the prices for my raw materials and postage going up I'll soon have to revise my prices and I'm debating whether I should build the shipping costs into my prices. Regardless of what I decide in the end, free shipping will remain a valuable promotional tool for me and other online retailers.
* In my opinion this should be all sellers. If you don't have a high enough profit margin to be able to afford to discount either for a sale or for wholesale buyers then you don't have enough profit built in to adequately reinvest in the business and pay yourself a reasonable amount for your time.