I present a unified approach to calculating the next-to-next-to-leading order (NNLO) soft and virtual QCD corrections to cross sections for electroweak, Higgs, QCD, and SUSY processes. I derive master formulas that can be used for any of these processes in hadron-hadron and lepton-hadron collisions. The formulas are based on a unified threshold resummation formalism and can be applied to both total and differential cross sections for processes with either simple or complex color flows and for various factorization schemes and kinematics. As a test of the formalism, I rederive known NNLO results for Drell-Yan and Higgs production, deep inelastic scattering, and W+ production, and I obtain expressions for several two-loop anomalous dimensions and other quantities needed in next-to-next-to-leading-logarithm (NNLL) resummations. I also present new results for the production of supersymmetric charged Higgs bosons; massive electroweak vector bosons; photons; heavy quarks in lepton-hadron and hadron-hadron collisions and in flavor-changing neutral current processes; jets; and squarks and gluinos. The NNLO soft and virtual corrections are often dominant, especially near threshold. Thus, a unified approach to these corrections is important in the search for new physics at present and future colliders.