In my years of doing work, I've accumulated a number of assets and resources that have helped me and that I know will help others. As a note, assets available are either Free Use or CC0 so they can be used in your own personal works legally. All links provided are services or tools I have personally used and have experience with. If there is a notable program that is not listed here, it's because I have not used it and cannot vouch for it.
Something that is important but doesn't fit any of the other categories is Artist Beware. A site that serves as a place to warn both artists and clients about scams, bad business, and potential bad deals in the landscape of personal commission work. It also hosts a forum for talking with and artists and clients alike for advice along with resources and articles on running freelance commission work.
Subtle Patterns: A nice collection of textures suitable for backgrounds for almost anything.
Pattern Cooler: Generate all types of patterns using your own color choices.
Pexels: CC0 Stock photos and video database.
CC0 Textures: A database of textures for CG applications.
Unsplash: Like Pexels, another CC0 stock photo database.
& Discover: A collection of fonts for free and paid.
Exljbris Font Foundry: Both Free and paid fonts.
Tyodermic Fonts: All fonts offered, free or paid, are permitted for commercial use.
Blambot: Resource of comic focused fonts and lettering how-to.
Volkron Typeface: A Single serif font but exhaustive in it's compatibility.
Google Fonts: A large directory of open type fonts for commercial use and embedding.
The League Of Movable Type: A collection of fonts by various designers all listed under the Open Font License.
Beautiful Web Type: A collection of assorted open source fonts.
Font Squirrel: The fonts found on this site are all free for commercial use.
Line of Action: Great resource for all kinds of gesture sketching exercises.
Elftown References: Old site with a large collection of reference images of all sorts of things.
Adorka Stock: A robust collection of photo stock images for use or referece.
Animal Photo Finder: A tool that helps you find reference images of animals based on the rotation of the skull.
Jetpens: The best place to purchase any sort of drawing or writing tool.
Medibang Paint: Free digital art program, for all platforms, including mobile. Provides cloud saves and free assets.
Krita: Free digital art program for all operating systems with tools and features comparable to Photoshop.
Fire Alpaca: Free digital art program for Windows and Mac, very simple with minimal features.
Sketchbook: Free digital art program, works best for sketches with tools intended to emulate real mediums.
Graphics Gale: Free pixel art program for Windows only.
Aseprite: 15$ pixel art program for all operating systems.
Affinity: While they provide other applications, their Designer application I use the most and is the best Vector application for the price.
Brackets: Free text editor. Has auto-fill, image and color previews with customizable color themes.