I've been making games for a very long time at this point, so here are the game development articles and resources that have helped me the most over the years. I kept putting off writing this post because I was worried I'd accidentally forget a really good article or two, but I can always just go back and edit them in when I inevitably remember them later.
Anyway, here they are, roughly organized by topic. I hope they help you as much as they helped me!
Gameplay programming
Sonic Physics Guide - A resource on the Sonic Retro wiki that explains how the classic 2D Sonic the Hedgehog games implemented their movement and physics code. It does a great job of demystifying the whole process, especially for people who have only ever coded platformers using a physics engine's built-in capsule colliders and so on.
Just play games and study them frame-by-frame, either by recording your gameplay and watching the video footage or by using an emulator with a Frame Advance feature. It's an important resource for seeing how things work!
Production and marketing
Stress-Free Game Development: Powering Up Your Studio With DevOps - A talk by Butterscotch Shenanigans' Seth Coster on how the studio completely turned around their terrible production pipeline. By far the best game production resource I have ever seen.
4 signs your publisher is stealing your money A talk by Landfall's Petter Henriksson on why and how you should make games without a publisher.
How To Make A Steam Page (free, signup required) - a series of video tutorials on how to set up your game's Steam page. There are all sorts of unwritten rules on how to do this (e.g. the order of your game's screenshots is very important!) so it's super helpful to have them all laid out.
Worthy festivals for indie games - A list of events that you can submit your indie game to. As a solo developer without much time to spend on marketing, online-only indie game events are a godsend. The list doesn't seem to be quite up to date (e.g. you can submit to INDIE Live Expo right now but the doc doesn't mention it) but I don't know of any better resources for this.
Pixel art
Pixel Art: Common Mistakes - An article by Derek Yu about some common pixel art mistakes and how to correct them.
Just look at other games' pixel art very closely. Pixel art is pretty low resolution and it doesn't require much physical technique. If you like something about a certain style of pixel art you can pretty easily just zoom in on an image, look how it works, copy it, and go from there. You don't have to learn which brushes they use or practice drawing smooth lines or anything like that.
Game design
ChoRenSha68k Development Notes - Notes from the developer of the popular shmup Cho Ren Sha 68k. It's very informative!
Shmup Workshop - A video series by @boghog on bullet hell shmup design, based on his bullet hell shmup design 101 document.
And finally, the most underrated resource of all: play a lot of interesting games and think about them. So many people out there have already experimented with all sorts of game ideas, so play the games that they made! If I wanted to make a great new 2D platformer, I think I would be better served by playing and understanding a wide range of interesting 2D platformers (e.g. Umihara Kawase, Ghosts 'n Goblins Resurrection, 1001 Spikes) instead of just playing Mario and Celeste and watching some youtubers talk about Mario and Celeste.
If you're a solo developer, I especially recommend playing other solo-developed games! Preferably ones that were developed in a reasonable timeframe, not decade-long passion projects. A big part of making games by yourself is figuring out which corners you can cut compared to games made by larger teams, and looking at other solo developers' games is a great way to learn that. You can learn a lot about shmup design from great games by companies like CAVE, but you'll learn a whole lot more about how to actually finish making a shmup from games like Kalikan, the Touhou series, Cho Ren Sha 68k or Like Dreamer.
It's even more important to actually make games, of course! I learned much more from making and releasing games than I would have done from just thinking really hard about making games and never doing it.
