1. A computer-implemented method comprising: receiving, at a server computer system, a first digital image data and virtual tag data to be associated with a real-world object in the first digital image data, wherein the first digital image data is captured by a first mobile device, and the virtual tag data includes metadata received from a user of the first mobile device; generating a first digital signature from the first digital image data that describes the real-world object; in response to the generation, inserting in substantially real-time the first digital signature into a searchable index of digital images; storing, in a tag database, the virtual tag data and an association between the virtual tag data and the first digital signature inserted into the index of digital images; receiving a second signature generated from a second digital image data captured by a second mobile device, the second digital image data includes one or more real-world objects; and transmitting the virtual tag data, received from the user of the first mobile device and associated with the real-world object in the first digital image data, to the second mobile device when a search of the index of digital images based on the second digital signature results in a match with the first digital signature.