Here it is and it is in fact a separate area of the site. While one can search for anything, results are organized in content-specific areas on Google Base (e.g., Cars, Jobs, Real Estate, Recipes, etc.). But depending on what the items are they may also appear in other Google sites: i.e., Google.com results, Froogle and Local.
Here are "cars" for sale. Here are "digital cameras" (interestingly with data provided by StepUp). Here are "'jobs" (with some listings from CareerBuilder). There are also some real estate listings from the NY Times.
Right now there's not a lot of content in Google Base (Recipes was the most complete of the categories I examined), but that could change fairly quickly. This is a very interesting but currently undeveloped product. Right now, it's not going to trump Craigslist or eBay or Shopping.com or vertical sites. And it may never; it will depend on how the user experience evolves over time.
The consumer experience and the content offered will be critical. If the content isn't there (especially local/offline content in many of these categories) then consumers won't use it. I continue to believe that a structured user experience that creates and fulfills expectations regarding what content is present and how to access it is crucial to the user side of the equation here.
But, then again, is it a consumer destination or a content acquistion strategy or both? It's really both — maybe with emphasis on the latter, which is primarily why it's free to use. And, in fact, that's where the site's long-term value may reside for Google if it becomes a direct channel or content/listings acquisition tool for distribution to other Google sites.
Finally, content "labeling" (as Google calls it) or tagging as others label it opens the door to a community layer (a la Yahoo!'s strategy). We'll see if that ever develops.
Here are the Google Base FAQs.