Most standard books on quantum mechanics discuss interference in the two-slit experiment and argue that if you turned down the intensity of the source sufficiently you would find that photons basically interfere with themselves, and not with each other. Paul Dirac himself wrote that "interference between two different photons never occurs". However, it now seems that this is not quite true, and that entangled photons can produce interference patterns that beat the diffraction limit expected for single photons.
Philip Walther of the University of Vienna and colleagues have shown that linear optics can be used to entangle N photons and get them to act as if they were just one entity with a wavelength N times smaller (Walther et al. 2004). Similar work has also come out of the University of Toronto from M W Mitchell and co-workers (Mitchell et al. 2004). So far, N = 4 is the biggest N that they have managed, but there is no obvious limit to how many photons can be entangled. This technology might even find commercial applications ranging from high-density optical storage to quantum computing.
Two entangled photons can be far away from each other and still yield correlated results for measurements that naively would seem to violate some combination of reality and locality - the famous Einstein-Podolsky-Rosen effect. A more exotic scenario, and one that has recently been measured, is when a photon hits a beam splitter and turns into what are effectively two versions of itself in two different places, which display the same sort of correlations between its two manifestations. Björn Hessmo and colleagues at the Royal Institute of Technology in Kista, Sweden, have managed to make delicate phase measurements of the two photons, which were really just one photon going two ways (Hessmo et al. 2004). They find that once again the bizarre predictions of quantum mechanics really do hold in our universe.
Further reading
B Hessmo et al. 2004 Phys. Rev. Lett. 92 180401.
M W Mitchell et al. 2004 Nature 429 161.
P Walther et al. 2004 Nature 429 158.