iquilezles.org — I highly recommend all materials and articles on this website, created by Inigo Quilez.
Articles include very useful materials such as 2D SDF and 3D SDF, intersectors functions for analytic raycasting, smoothstep like functions, and many others worth checking.
shadertoyunofficial.wordpress.com — lots of useful tips and materials about Shadertoy and shaders, created by Fabrice NEYRET.
Usual tricks in Shadertoy some basics and not very obvious tips, Special Shadertoy features info about Shadertoy variables and resources format.
Shadertoy media files Shadertoy resources like images and others.
(Shadertoy audio texture more detailed explained here)
Avoiding compiler crash and Compatibility issues in Shadertoy and webGLSL.
hg_sdf — 3D SDF library.
Khronos OpenGL-Refpages — list of all GLSL functions with description and limitations.
SPIR-V Extended Instructions for GLSL (Vulkan) — list of GLSL functions for Vulkan.
Shadertoy alternative and similar websites:
vertexshaderart.com — WebGL particles shaders.
neort.io — WebGL and also allows uploading HTML and video content.
glslsandbox.com — WebGL fragment shaders.
For more complex projects — itch.io or newgrounds.com or other similar services can be used, a service that allows uploading web projects, even github/gitlab pages that have a similar purpose. For the offline web-template look below.
Offline Shadertoy related applications:
For my offline shader development — text editor with shader-viewer/render is enough, complex software has way too many bugs to deal with.
vulkan-shadertoy-launcher — very minimal application to render shadertoy shaders, using C with Vulkan API. Support Linux and Windows.
shadertoy-to-video-with-FBO — python script, rendering Shadertoy shaders to video frame by frame, using OpenGL. Support Linux and Windows.