Database of optical constants http://refractiveindex.info/
Other projects interfacing with the refractiveindex.info database
https://github.com/polyanskiy/refractiveindex.info-scripts
Collection of Python scripts related to refractiveindex.info database
https://github.com/kitchenknif/PyTMM
Database browser and Transfer Matrix Method implementation (by Pavel Dmitriev)
https://github.com/HugoGuillen/refractiveindex.info-sqlite
Python 3 + SQLite wrapper for the refractiveindex.info database (by Hugo Guillen)
https://github.com/cinek810/refractiveindex.info
Support for operations not possible through the web interface, e.g. find materials with refractive index within a specified range (by Marcin Stolarek)