Heropatterns!
If you take a peek in the background here, you'll notice a cool and funky background. On this page, it's a circuit board. On the home page, there's a whole topological pattern there, too.
What you're seeing is called a "SVG", or a scalable vector graphic. how that differs from your bogstandard png is that no matter how big you make it, there is no loss in quality. I.e. no weird pixels gunking up the whole thing.
Heropatterns takes that, and makes pure css snippets of code you can simply copy/paste into repeatable backgrounds. "Repeatable", because there's no obvious point where the background "starts" and "ends".
I just think it's really cool though that it's completely free... I really appreciate when people do stuff like this. I feel like where companies try to put everything behind a paywall, I'm glad there's people out there that are kindhearted enough to share with everybody. Coding, to me, is a hobby that anybody can start learning. I've always appreciated that there are sources almost literally everywhere to help you learn. Coding somehow has a super elitist community, but also has this other side of people who aren't afraid to say "Yeah, I've been there lol." Which is to say, I always feel like people who put their stuff out for free are just doing the community a really good service. I don't know, I'm rambling lol.
Check it out!