He put together an proxy service he calls GoogleSharing, that aims to anonymize all your searches and movements inside and from Google online services that don't require you to login into your Google account. That means that it can anonymize your use of Google's search engine or Google Maps, but can't be of use when you use Gmail, Google Calendar, Docs, Photos, and all other services you have to log into to use.
According to Marlinspike, Google knows "who your friends are, where you live, where you work, and where you spend your free time. They know about your health, your love life, and your political leanings. These days they are even branching out into collecting your realtime GPS location and your DNS lookups. In short, not only do they know a lot about what you're doing, they also have significant insight into what you're thinking". How's that for a wake-up call?
So, how does GoogleSharing work? In short, the service mixes your requests with those of other users, making it impossible for Google to identify which are truly yours.
All you have to do is install an Firefox add-on and every time you make a request to any of those services, it is redirected to a GoogleSharing proxy, where it is stripped of all identifying information and replaced with the information from a GoogleSharing identity. The request is then forwarded to Google, and the response comes back to you also through the proxy.
Forbes reports that Marlinspike has added another layer of security to the searches that go out through GoogleSharing: all the requests are sent via HTTPS, so that if there is some malicious software on the user's computer, it can't intercept the information.
He is also aware that some people might not trust him with the data in question. For them, he offered the service's code to use for setting up their own proxy.