The Rydberg series (1s 2-1snp) 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 1s 2 2s-1s2snp and 1s 2 2p-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 (± .5 mÅ) have been determined by comparison to neighboring reference lines from Hand 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.