I think, for atomic lines the NIST database is quite good, for molecules the data are scarce, however.

http://www.nist.gov/pml/data/asd.cfm

For atomic (neutral and ions) lines the NIST data are the gold standard. For molecular transition of molecules present in air the HITRAN database is very good:

https://www.cfa.harvard.edu/hitran/

definitely NIST

if you prefer a traditional book, for offline access, I believe a lot of useful spectra are published in the CRC Handbook of Chemistry and Physics

Thank you all for your help and opinions.  I will look into all of your suggestions.

Regards,

Bernardo.

The HITRAN database is quite good for molecular spectra (of common molecules).

Generally the NIST database is the most used.

Yes, http://www.nist.gov/pml/data/asd.cfm contains a lot of data.

Maybe this database will be helpful

NORAD-Atomic-Data (Nahar-OSU-Radiative-Atomic-Data)

http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.html#AtomicDataTbl

You can also take a look on http://www.nucleide.org/

You can also take a look on http://plasma-gate.weizmann.ac.il/directories/databases/