Spectral Data Base for Organic Compounds (AIST): "SDBS is an integrated spectral database system for organic compounds which includes 6 different types of spectra under a directory of the compounds."
ChemSpider (Royal Society of Chemistry): "ChemSpider is a free chemical structure database providing fast text and structure search access to over 58 million structures from hundreds of data sources." Includes literature references, physical properties, interactive spectra, and chemical suppliers.
LibreTexts Organic Chemistry (UC Davis): "The Chemistry LibreTexts library is a principal hub of the LibreTexts project, which is a multi-institutional collaborative venture to develop the next generation of open-access texts to improve postsecondary education at all levels of higher learning. The LibreTexts approach is highly collaborative where an Open Access textbook environment is under constant revision by students, faculty, and outside experts to supplant conventional paper-based books." This open access textbook focuses on Organic Chemistry.
NIST Chemistry WebBook (National Institute of Standards and Technology): "This site provides thermochemical, thermophysical, and ion energetics data compiled by NIST under the Standard Reference Data Program."
IUPAC Periodic Table of the Elements (2016): "The latest release of the Periodic Table (dated 28 November 2016) includes the recently added elements 113, 115, 117, and 118 with their names and symbols"
Organic Nomenclature (William Reusch of Michigan State University): An introduction to the IUPAC system of nomenclature for organic compounds.
Atomic Orbitals (Michael W. Davidson and Florida State University): "This tutorial examines the first four energy levels of an atom, s, p, d, and f, chosen through the pull-down menu. By selecting a set of orbitals, you can select any combination of orbitals, using the radio buttons, to view all orientation configurations of these electrons based on the number of electrons located in each energy level."
Periodic Table of the Elements (Los Alamos National Laboratory): Provides not only the periodic table, but also detailed information about each of the elements. Just click on the element in the table to find more information.
Randall Monroe via xkcd.com (Creative Commons)