Google Base is yet another beta service of the popular search engine. It allows users to find things based on predefined attributes instead of keywords and to search through content that might not be on any website.
There are many item types available for posting to Google Base, but my experience has been with Products because of its importance in the e-commerce industry. Base is significant not because people search it directly, but because Google Product Search, formally know as Froogle, retrieves its results from it. Product Search is a useful tool to e-commerce because it allows retailers to advertise their products for free. From personal experience I have found that Product Search can bring more traffic to an e-commerce site than an expensive Adwords Campaign.
Product Search is unusual because it searches through the afore mentioned predefined attributes and displays results based solely on matches. Fees and commissions are not accepted so they can not influence the output. This can help even the playing field for even the smallest of online retailers. Products are defined by attributes (some required, some optional) and are entered either one at a time, through a number of bulk formats including XML and CSV, or by using their API.
Because Google Base/Product Search offers a virtually unlimited amount of free advertising, I would strongly recommend its use to any online store looking for more traffic. Be warned, it takes more to implement than a basic Adwords Campaign. I suggest finding a method to create entries in bulk or using their API because submitting individual entries would be prohibitively time consuming in many cases.