Google Dynamic Remarketing
Google Dynamic Remarketing lets you create customized ads for specific products in your catalog. Shoppers will then see these ads when visiting websites on Google's Display Network or performing a search on Google, reminding them to come back to your website to make a purchase.
Anaraky GDRT for Magento
The free Anaraky GDRT for Magento extension allows you to integrate the Google Dynamic Remarketing Tag into your store's product pages. The extension is easy to install and does not change any cored files in Magento. You can find it in Magento Connect: