If I need a quick background pattern to spruce something up, I often think of the CSS3 Patterns Gallery. Some of those are pretty intense but remember they are easily editable because they are just CSS. That means you could take these bold zags and chill them out.
I also like the new CSS Pattern website. It has some pretty sophisticated backgrounds that are done just with CSS. Very easy to copy/paste.
My usual go-to through is Hero Patterns. They are also editable, but they already start from a pretty chill place, which is usually what I’m looking for from a pattern. They also happen to provide ones we’ve baked into the Assets Panel on CodePen for extra-easy access.
If you’re into SVG-based patterns (and who isn’t?) SVG Backgrounds has some extra clever ones. Looks like it’s gotten a nice design refresh lately, too, where the editable options are intuitive and the code is easy to copy. If you are a DIY type, remember SVG literally has a <pattern> element you can harness.
I’ve seen some new fun pattern sites lately though! One is the exceptionally deep Tartanify which has over 5,000 Scottish tartan patterns. Paulina Hetman even wrote about it for us.
Beautiful Dingbats has a very nice pattern generator as well that seems pretty newish. It’s got very fun controls to play with and easy output.
One that is really mind-blowing is Mazeletter. It’s a collection of nine fonts that are made to be infinitely tiling, so you essentially have unlimited pattern possibilities you can make from characters.
Just to end with a classic here… you can’t go wrong with a little noise.
https://www.toptal.com/designers/subtlepatterns/
https://transparenttextures.com/
https://superdesigner.co/tools/css-backgrounds