The reported interstellar and circumstellar molecules.

NRAO National Radio Astronomy Observatory

A GSFC List including Ices.

A Koln List.

Resources for Frequencies

Splatalogue .

NIST Recommended Rest Frequencies for Observed Interstellar Molecular Microwave Transitions by Frank J. Lovas

NIST Physical Reference Data .

Molecular Spectroscopy Home Page (Includes transitions not yet observed in the ISM) by Herb Pickett

The Cologne Database for Molecular Spectroscopy additions.
 
 

The Astrochemistry Working Group of the International Astronomical Union home page lists other resources