A Koln List.
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
Database for Molecular Spectroscopy additions.
The Astrochemistry Working Group of the International Astronomical Union home page lists other resources