Disconnected diagrams give crucial contributions to the physics of flavor singlet hadrons and to scalar form factors of non-singlet hadrons. Naive lattice calculation of the disconnected diagrams, however, requires a huge number of fermion matrix inversions and hence a prohibitively large computational cost. In this article, we present recent studies of the flavor-singelt meson spectrum and nucleon strange quark content using the all-to-all propagator to calculate the relevant disconnected diagrams.