A while ago, it was docker. It really makes it easy to isolate and bootstrap one's architecture.
More recently, it was continuous integration/deployment/delivery. I used to be skeptical about it since I couldn't see myself trusting the results but once I set up push-to-deploy, at least, I haven't looked back.
And, pretty much a month ago, I decided to stop resisting the cloud, which I also found to be a huge boost. It turns out that, while I didn't hate managing architecture (in production), I didn't particularly enjoy it either and I'd much rather write code. And thanks to app engine (the cloud provider I chose), and some help from Gitlab CI, I can do that almost exclusively now.