First, let’s define what a product shopping search site is. What differentiates these sites is that they’ll take a product feed from your site, and make the listing available to the public. Potential customers who are actively looking to buy products will go to one of these sites, PriceGrabber for example, and type in a product or category. The engine returns the best product matches and links to the stores (your store!) to purchase. The merchant usually pays a small fee per click.

The merchant (you) submit your products generally through a feed file or via a web service. You’ll want the feed to be automated, and the feed will need to be formatted per the product search engine’s specifications. There are both extensions and services that can do the complex work of taking your Magento data and submitting it in the required format. More on the options below.
These search engines offer a variety of tools for the merchant to target their ads, budget and report. All of these toolsand services can be integrated with Magento CE or EE.

Shopzilla – They’ve been around for more than a decade, haven’t changed their name, and are still an excellent product search site. A smaller name, but still an excellent choice if you’re looking for more options to maximize your e-commerce search best practices.

Product Feed from Amasty ($129)Feeds into 11 product shopping sites and marketplaces.

Google Shopping Feed from RocketWeb ($199)Only works with Google, but works perfectly in our experience.

Data Feed Manager from Wyomind ($145)Works with dozens of product shopping and marketplace sites. More complex, but can be modified to work with the largest variety of sites.
Dynamic CreativeThey have an extension that integrates with Magento. This does more than feed products into Google; it also supports Adwords campaigns and remarketing on Google.

GoDataFeed (from $59/mo)Integrates with over 100 product shopping search sites and marketplaces. They have a free extension for Magento.