NIST: Atomic Spectra Database - Energy Levels Form

Peter van Hoof's Atomic Line List

Nebular Analysis Software

Catalog of mostly long-wavelength/low-frequency transitions,
tailored for ALMA

Up-to-date list of detected IS molecules,
part of the Cologne Database for Molecular Spectroscopy (CDMS)

NASA Goddard list of Interstellar molecules and ices
-also hosts a database of infrared spectra and optical constants
for dust constituents