X-ray Free-Electron Lasers (XFELs) are unique sources of high brightness coherent radiation. However, existing XFELs supply only linearly polarized light, precluding studies of chiral dynamics. A device called the Delta undulator has been installed at the Linac Coherent Light Source (LCLS) to provide tunable polarization. With a reverse tapered planar undulator line to pre-microbunch the beam and the novel technique of beam diverting, hundreds of microjoules of circularly polarized X-ray pulses are produced at 500-1200 eV. These X-ray pulses are tens of femtoseconds long, have a degree of circular polarization of 0.98 0.02/−0.04 at 707 eV, and may be scanned in energy. We also present a new two-color X-ray pump X-ray probe operating mode for LCLS. Energy differences of ΔE/E Show Partial Abstract