Some resources I really like:
Tools / Resources
Feedly - Online RSS reader, great for keeping track of the latest literature.
Chemistry reference resolver - An invaluable site, just copy a citation (in almost any format), search and it will take you directly to the article. In most browsers, you can set it as a search engine too.
Physical chemistry conferences - A list of upcoming chemistry conferences, regularly updated.
eCoupling Server - A website that uses the Generalized Mulliken Hush or Fragment Charge Difference approaches to derive diabatic states and coupling elements from adiabatic electronic structure calculations directly from the outputs of some popular quantum chemistry codes. Here's the associated paper.
Solvent properties - This is a list of solvent dielectric constants (εrel) and refractive indexes (nD) for a vast array of solvents (>700). Really useful for specifying unusual solvents in a calculation that utilises continuum solvation.
Quantum chemistry notes - I wish I had found these whilst I was an undergrad. Extensive but eminently readable notes from Prof. David Sherrill.
Chris Cramer's Comp. Chem. lectures - This set of videos from Chris Cramer has a set of lectures he uses to teach a course at the University of Minnesota. The pacing is great and he introduces all of the core elements of computational chemistry. There are also some associated notes/slides available here.
Oxford Quantum Mechanics lectures - A fantastic set of recorded lectures from Oxford. As a chemist, it's enjoyable to experience QM from a physicist's perspective.
Mariapfarr Workshop slides - These workshops cover a range of advanced theoretical chemistry topics, from relativistic quantum mechanics (2012), to heterogeneous catalysis (2008) in quite a lot of detail.
The Essence of Linear Algebra - If you're like me and didn't do much in the way of matrix mathematics at school, you might enjoy this series of short YouTube videos which tries to provide viewers with an intuitive understanding of concepts like eigenvectors and basis functions.
Fundamentals of Data Visualization - For me, the most important parts of papers are the figures, but it's difficult to design a good one. This book is a great primer, containing simple and clear principles for how to visualise complex data. (HT: Grant Hill)
LaTeX wikibook - If you're new to LaTeX, or want to look at some documentation, this is the place to go.
LaTeX StackExchange - The answer to any question about LaTeX can be found here. If you have a problem with LaTeX, you'll find the answer here!
detexify - This is a cool little site where you draw in a symbol and it tells you the LaTeX command to get it, plus any dependencies you might need.
Known Chemistry Packages - A reasonably comprehensive list of chemistry-related packages for (La)TeX.
coolors.co - This is an online colour palette generator. I've used this quite a lot for picking colours for use in graphs.
Obligatory units converter - Computational chemists always seem to include a link to a units converter, so I'm bucking the trend by only providing one that converts one type of unit, because.