Project Wonderful (www.projectwonderful.com) - This is a unique advertising tool that is not free, but can be very cheap. Basically, you deposit money from your Paypal account to your Project Wonderful account. You create an ad and then bid to place your ad on various blogs and websites based on criteria you choose. The concept is a bit abstract, but you can often advertise on some smaller blogs and sites for very little. Worth checking out.
These are just a few of my favs, but if you want to see much, much more...take a look at this list of 50 FREE sites to promote your shop: http://www.createapendant.com/sitepromote.htm