ECP SDK projects drive work on the Extreme Scale Scientific Software Stack (E4S). E4S exists to accelerate the development, deployment and use of HPC software, lowering the barriers for HPC users. E4S provides containers and turn-key, from-source builds of more than 80 popular HPC products in programming models, such as MPI; development tools such as HPCToolkit, TAU and PAPI; math libraries such as PETSc and Trilinos; and Data and Viz tools such as HDF5 and Paraview. Key E4S initiatives include porting and improving interoperability between E4S packages in pursuit of ECP objectives.