The QCD axion as a solution to the strong CP problem together with the properties of axions in string theory suggest the simultaneous presence of many ultralight axions. Conversely, the presence of such a plenitude of axions (an "axiverse") would be evidence for string theory, since it arises due to the topological complexity of the extra-dimensional manifold and is ad hoc in a theory with just the four familiar dimensions. These axions give rise to a number of distinctive observational signatures, including the rotation of the CMB polarizations at a level within reach of the Planck satellite, steps in the dark matter power spectrum, and spindown of astrophysical black holes. The same string theory mechanism suggests also the presence of a plenitude of massless vector bosons. In the presence of low energy supersymmetry, their superpartners--the photini---extend the MSSM neutralino sector and lead to interesting and unusual LHC signatures.