Google has opened up an early access program to keep eager users updated on what experimental developer house Area 120 is up to, and to give them chances to sign up for things that Area 120 is offering to consumers. The signup will net you an email whenever something new is ready to be trotted out, and in most cases, that email will contain a link allowing you to sign up and check out the newest experiment. The signup form simply asks your email address and mobile OS of choice, giving the choice between Android, iOS, and "Other".
Presumably, the "Other" option in the mobile OS choice menu for signing up means that Area 120 could eventually roll out experiments that can be used across platforms, such as in a browser or via the cloud. Of course, this could also cover initiatives targeted at developers, such as a VR ad system that comes as a plugin for Unity. Thus far, Area 120's initiatives that have reached the public have included the aforementioned VR ad system, a social YouTube client called Uptime, which is sadly exclusive to iOS for now, and the voice-based Supersonic Fun Voice Messaging app on Android. All of these initiatives are now available to the public, and as of this writing, Area 120 has not announced any other projects in the works.
Area 120 is an idea incubator within Google that allows employees to iterate ideas quickly. Google allows the employees to work on whatever they want, and gives them the funding and manpower to back it up. The catch is that it all has to happen very quickly. If this sounds similar to the Moonshot lab, that's because it is. The quick iteration and tossing failed projects just as quickly means that, unlike with the Moonshot lab, CFO Ruth Porat won't have to bring the hammer down and cut spending in the department. Googlers are allowed to spend 20% of their company time on side projects most of the time, but in Area 120, they can not only spend their entire work day on those ideas, but they also have Google's full might behind them.