Assets & References

Textures & Backgrounds

Transparent Textures	This site's background is based off of a pattern from here.
Doodad Pattern Generator	Tool for creating seamless patterns. Royalty-free.
Subtle Patterns	Exactly as it says on the tin
Mintyoh's Scans	Scans of origami paper and similar. Definitely restrict to hobby usage.
Low Poly Generator	Makes mosaic-like images from either a pallete or an uploaded image.
Salvaged dot Nu	Textures, brushes, & similar from old personal websites that no longer exist. Another one to keep to just hobby usage.
Appendix Squared	Textures, brushes, & similar. Usage rules are here.
Texture Labs	Textures, brushes, & similar. Mixture of paid and free, paid is mostly bulk downloads and brushes/effects. Usage rules are here.
Texture Ninja	CC0 Textures and stock images.

Color Palettes

Coolers	Color palette generator, with browsable color palettes.
Colour Lovers	A similar, older website. Has a pattern generator!
Adobe Color	Adobe's answer to the above. It's free though, shockingly.
Paletton	Another color palette generator. No user provided palettes on this one.
Design Seeds	Blog of color palettes based off of photos.
Color Palette Spreadsheet Mayhem	A large amount of collections of color swatches and palettes converted into a spreadsheet.

Music & Sound Effects

Audiostock	Japanese site. To quote the Tumblr post this was yanked from "you can literally just license a soundtrack from audiostock for under 10k yen and it'll immediately give your work a significantly higher production value than using stock rpgmaker music".
Dova-Syndrome	Another Japanese site.

Stock Photos

People

Adorkastock	Formerly known as Senshi Stock. Focused on dramatic poses, especially ones useful for fantasy art. Usage rules are here.
Nappy	"Beautiful photos of Black and Brown people, for free. For commercial and personal use."

Clip-art

unDraw	Very corporate looking, but available for pretty much any usage. "unDraw grants you an nonexclusive, worldwide copyright license to download, copy, modify, distribute, perform, and use the assets provided from unDraw for free, including for commercial purposes, without permission from or attributing the creator or unDraw."
Noun Project	Icons AND stock photos. The terms of use covers usage rules. Partially paid.

Marketplaces

Many of these provide free assets, either having some things free year round, or occasionally discounting things to $0. Many will require your e-mail and then spam you, so be warned.

Indieground	If something is in the freebie category but lists a price, that means that there's both free and paid licensing options available. (The free licensing option is likely personal use only, and may not include all assets)
DLSite	Rather than just selling doujin works, DLSite also sells assets and tools for doujin works. This link will take you directly to the tools/assets category with some sensible filters. IF you go to the main page you will probably see boobs, so don't go there in public lmao. This site definitely won't spam you.