Google Now is one of many new features of Jelly bean the latest incarnation of the Android OS. In some ways it initially seems similar to Apple's Siri, but it goes further. Rather than just answer questions or enter into a knowledgable conversation it remembers things about you and offers information even when not prompted.
On the launch page Google show various examples of how Google Now will help users. All but one example are simple search, location based searches, local weather, places to eat etc. But one feature stands out.
What is most interesting about Now is that it can use different data sources in intelligent ways. As Hugo Barra said it is designed :
"to figure out what information you might need, and when"
For me they only showed one truly new example of this. Google Now knows what is in your diary and if the diary entry includes a location it will compare this with your current location and prompt you about when you need to leave to arrive in time for your appointment. It does this by selecting a route and knowing current traffic congestion.
While this is not rocket science in terms of a useful function, and it has been the staple of visions of future tech for some time, this is the first real world incarnation of this functionality. This form of merging data to form useful information is one of the goals of the semantic web. I assume the plan is to marry Now to Google's semantic search and use a wider range of more diverse data sources as input. If this is the case then the widespread unconscious use of the semantic web will have arrive sooner than expected.