The Rydberg series (1s2 to 1snp) up to n = 14 of helium-like argon (Z = 18) has been observed from Alcator C-Mod plasmas using a high-resolution X-ray spectrometer array. High-n satellites to these lines of the form 1s22s to 1s2snp and 1s22p to 1s2pnp with 3 n 12 have been recorded. X-ray spectra of 2l - nl' transitions with 3 n 18 in molybdenum (Z = 42) and 3 n 12 in krypton (Z = 36) and niobium (Z = 41) from charge states around neon-like have also been measured. Numerous examples of the configuration interaction, which alters the line intensities in some transitions of neon-like ions with nearly degenerate upper levels, have been observed. Accurate wavelengths of all of these transitions (±0.5 mÅ) have been determined by comparison to neighboring reference lines from H- and He-like charge states. Line identifications have been made by comparison to ab initio atomic structure calculations, using a fully relativistic, parametric potential code. Measured line intensities have been compared with collisional radiative modeling that includes the contributions from dielectronic recombination and inner shell excitation rates, with good agreement.