It’s worth noting that google is an amazing resource for a huge amount of content on any area of tabletop gaming and this shouldn't be overlooked, but these are sites I’ve found particularly useful or informative:
Board Games
Board Game Geek - The ultimate board game reference site - this list should end here cos they link to pretty much everything else!
The Dice Tower - The team provide a great amount of coverage on all areas of gaming with great reviews on their youtube channel (assuming you can handle the component drop)
Rahdo Runs Through - Not always the slickest production but Rahdo provides great instructional videos
Shut Up and Sit Down - Reviews that are more like a comedic dramatization, but great fun
Tabletop Simulator 101 - A great guide to putting your games into a digital format
Tabletop Mentorship Program - Not exclusive to board games but have been really helpful in that regard
Cardboard Edison - Great source of intel and advice on making games
Game-Icons - Wonderful resource for useful icons when making games
Tabletop Publishers Dossier - Well maintained list of publishers and what they’re looking for
The Board Game Design Lab - I first discovered Gabe’s corner of the internet through the Facebook community who are a wonderful group, but he puts out a wealth of useful content and commentaries
Tabletop Game Designers Association - Newly launched association to support and advocate for game designers.
RPG Materials and Rules
Dungeons & Dragons - The official webpage for D&D, largely product information and adverts, but some useful content
D&D Beyond - Masses of resources for D&D 5e
En5ider - not only a great Patreon with tons of original and professional D&D 5e content, but they've also published some of my work
D&D Quick Reference - great clickable list of actions and conditions
Drive Thru RPG - A great digital store for everything RPG related including a good selection of free resources
Dungeon Masters Guild - Similar to Drive Thru RPG, but exclusively focused on D&D in partnership with Wizards of the Coast
Running the Game - A wonderful intro to GMing by Matthew Coville
DnD Spells - A great resource for putting together spellbooks and spell cards
Strolen's Citadel - This link goes straight to the classic '5 Room Dungeon' article, but there’s plenty more to discover here
Medieval Fantasy City Generator - Generates city maps with a number of details that you can set
Not Another Tavern Generator - Generates tavern names, owner and clientle descriptions and a menu for more taverns than you count
Lines and Veils - an excellent description of the use of lines and veils
Matt Mercer’s GM Tips - tips from Matt on how to be a better GM
How to be a Great Games Master - Straightforward advice on how to do better
Taking 20 - A great video channel about all aspects of D&D
CR Calc - A handy Challenge Rating calculator
The Monster Know Blog - a great resource for monster tactics
Consent in Gaming - A great resource for GMs looking for tools and advice for handling mature content and keeping the table safe for all players