Webelements, for information about all of the elements in the Periodic Table: https://www.webelements.com/
IUPAC Color Books, for nomenclature and other topics:
https://iupac.org/what-we-do/books/color-books/
Specifically for inorganic compounds, the Red Book:
https://iupac.org/wp-content/uploads/2018/05/Inorganic-Brief-Guide-V1-3.pdf
NIST Webbook, for physical properties of compounds:
https://webbook.nist.gov/
NIST Atomic spectral database, for information on all ions:
https://www.nist.gov/pml/atomic-spectra-database
The Orbitron, a great way to view atomic orbitals:
https://winter.group.shef.ac.uk/orbitron/
Otterbein College symmetry site, a great resource for symmetry and group theory:
https://symotter.org/
IONiC – Interactive Online Network of Inorganic Chemists /
VIPEr – Virtual Inorganic Pedagogical Resource – many educational resources on inorganic chemistry.
https://www.ionicviper.org/
Professional Societies
The International Union of Pure and Applied Chemistry (IUPAC)
https://iupac.org/
The International Society of Magnetic Resonance (ISMAR)
https://ismar.org/
The International EPR (ESR) Society
https://ieprs.org/
The American Chemical Society Division of Inorganic Chemistry (ACS-DIC)
https://acsdic.org/
The Society of Biological Inorganic Chemistry (SBIC).
https://www.sbichem.org/
The American Society for Biochemistry and Molecular Biology (ASBMB) https://www.asbmb.org/
(Bio)Chemical Structural Resources
The Cambridge Crystallographic Data Centre (CCDC), for structures of organic/organometallic molecules (license required):
https://www.ccdc.cam.ac.uk/
The Inorganic Crystal Structure Database (ICSD), for structures of purely inorganic systems (license required):
https://icsd.fiz-karlsruhe.de/index.xhtml
The Protein Data Bank, for structures of biological molecules (free site):
https://www.rcsb.org/
Chemical Suppliers
Sigma-Aldrich
https://www.sigmaaldrich.com/US/en/services
The first “go-to” supplier of chemicals for laboratory research.
Strem
https://www.strem.com/
My favorite supplier for inorganic/organometallic complexes, ligands.
Alfa-Aesar
https://www.alfa.com/en/
An excellent supplier of pure elements and simple inorganic complexes. They will become part of Thermo-Fisher as of 2023-07-31.
Chemical Industry Supplier Registry
https://www.chemicalregister.com/
Compilation of chemical suppliers of commodity and specialty chemicals by name or CAS registry number.