One place that I know is Google Product search, which is free and gives quite a lot of traffic when people are searching for products you sell.
What other ways are there to get free traffic to your e-commerce website?
A lot of it depends on the resources you have available, email is a great option if you have a list, social sites such as stumbleupon if you have the right content. The search engines can drive a lot of traffic, if your site is not yet ranking, you can take advantage of any applicable universal search features, in addition to the shopping results you already mentioned there are video, images, news results, realtime search if you're using twitter and more.
If you offer coupons there are quite a few coupon sites that let you list your deals for free. If you have unique products or are doing something news worthy press releases can be worthwhile. There are plenty of other options but hopefully this will give you enough to get started.
I wouldnt worry so much as to traffic but more about building your brand and actually getting sales. You want to focus on quality over quantity in my opinion.
Depending on what you are selling look for blogs, forums and other forums where you can share your site, and even advertise. If you are looking for "Free traffic" consider creating an affiliate program where you can share revenue from those marketing your products.
So if you are selling cameras, you could offer the operators of a popular photography site the option to showcase your site, or specific products. When their visitors click on the links then you could pay per lead or per actual transaction.
Also, focus on high customer service and repeat business.