Plugging my own tool, but I wrote a Chrome extension that checks websites for SEO, speed, and security best practices (it's a crawler so it can check 100s of pages in a few minutes):
It saved me a lot of time and caught tons of mistakes before they got to production while working on multiple websites as a contractor
For example, I was assisting a team on one sprawling dynamic website with a lot of SEO and performance issues - the site had been hacked together over the years such that editing one page would usually break groups of unrelated pages in unexpected ways. My extension helped me get on top of which pages were dependent on each other, to prioritise what was worth fixing, and to confirm improvements had been made without breaking anything.