Libraries of observed stellar spectra are numerous. They are often at the source of synthetic libraries. Their main characteristics are of course: the covered spectral domain, the spectral resolution, the covered spectral types and stellar properties (surface gravity, temperature, metallicity, abundance, ...), and we should also add the spectrophotometric quality (which flux calibration), since this is a critical issue for the modeling of the spectrophotometric emission of galaxies.
Here are a few illustrative examples of what exists today:
The IDS Lick stellar library which was used by the Lick collaboration to derive the famous "Lick indicex". This library is unfortunately not flux calibrated, but was used extensively, specifically for the derivation of the famous "Lick indices". See Worthey & Ottaviani, 1997 ApJS, 111, 377 and the web address: http://astro.wsu.edu/worthey/html/s...
The Jones stellar library which contains spectra for 684 stars observed with the Coudé Feed telescope and spectrograph at KPNO. The spectral domain covered is 3820 - 4500 Å and 4780 - 5460 Å at a resolution of 1.8 Å FWHM ( 60 km/sec). A paper by Leitherer, C. et al. (1996 PASP, 108, 996) describes the spectra. This library is kind of "oldish" (but more recent than the IDS library, and it is flux calibrated), but is at the source of the synthetic library by Alexandro Vazdekis (see http://www.iac.es/galeria/vazdekis/...).
The Indo-US library obtained which (according to the official web page) "consists of spectra for 1273 stars obtained with the 0.9m Coudé Feed telescope at Kitt Peak National Observatory. The spectra have been obtained at an original dispersion of 0.44 Angstroms/pixel, at a resolution of 1 Angstroms FWHM. To cover the entire wavelength range of 3460 Angstroms to 9464 Angstroms requires five separate observations with different grating settings." More details are available in Valdes, et al., 2004, ApJS, 152, 251, and the official web page is http://www.noao.edu/cflib/
The library by Leborgne, STELIB, available at OMP, France, at http://www.ast.obs-mip.fr/article18... See Le Borgne, et al., 2003, A&A, 402, 433. This library is, according again to the web page " an homogeneous library of stellar spectra in the visible range (3200 to 9500A), including stars of all spectral types, luminosity classes and metallicity that can be observed from the ground with the current instrumentation. Most of the stars in our sample have already well determined metallicities. The spectral resolution of our Stellar Library is about 3A FWHM. "
A very helpful though rather long (but not exhaustive) list of stellar libraries is maintained by David Montes and can be found at : http://www.ucm.es/info/Astrof/inves...