Happens to be the only shopping search engine which allows merchants to define their own attributes (optional fields). It is also formerly known as Froogle and if you have any product that can be sold online, you have no excuse to not use this service. It's free, and Google is trying to promote it's usage. If you want better results on the shopping engines, try optimizing your feed - it's no longer good enough to just post all your products and expect your listings to be found.
There is a new opportunity that can be had for online retailers, and it's known as the "Onebox" result. To learn more about this great way to drive targeted traffic, visit OneBoxer a fantastic blog that covers all angles better than I can.
Recently, Google made more requirements on a generic level, but there may be more if you are within a specific product category:
Understandibly, some merchants are upset at this since they have thousands of products that now need more information, and some products may not have all of the required fields (expiration date, for example).
So how can you take advantage of this?
Some people will search by color, some by material and others by style. All of these are fantastic opportunities to get your product in front of a consumer who is ready to buy. It will take some time to create and get all the tweaks worked out, but it is well worth it.