Literature

• ADS Abstract Service - Literature search
• astro-ph - Preprint archive

Astronomical Data

• DSS - Digitized Sky Survey
• NED - NASA/IPAC Extragalactic Database
• ISO - Infrared Space Observatory Data Archive
• Astrovirtel - Accessing astronomical archives as virtual telescopes
• RRFID - The Radio Reference Frame Image Database

Employment opportunities

• AAS Job register

Little programs

• Coordinate transformation (e.g. B1950 to J2000)
• Calendar converter (e.g. Gregorian to MJD)
• JPL's HORIZONS Ephemeris Generator (for solar system objects)

Molecular line frequencies

• Rest Frequencies for molecular transitions - The LOVAS catalogue from the National Institute of Standards and Technology
• CDMS - The Cologne Database for Molecular Spectroscopy

Other useful information

• Estimating Observing Times - written for the 30-m telescope, but imho also useful in a more general context.
• "Representative" Spectra for Heterodyne Observations - JCMT spectra of possible line calibrators.