SearchMage supports Google, Bing, Yahoo, DuckDuckGo, Yandex and Baidu. You can choose in options which search engines are enhanced.
The primary driver for this extension was the fact that especially when searching with Google and using "sort by date" there are A LOT of spam domain hits which lead to potentially malicious sites. It turns out that a simple regexp will catch most of them: \.php\?\w{6,9}=
So I first implemented functionality to be able to block these spam domains (and to easily report them to Google as well).
Then I found myself clicking many times different menus when I wanted to search by date (try it on Google for example). Those clicks add up so I wanted to make it faster and simpler. So I implemented the popup based search and made the UI as simple as possible but still powerful (sort by, search category). I later added support for multiple search engines to unify the experience.
Then I found myself often searching for certain types of things which are provided by a handful of websites. So I figured that having "search profiles" would be good so I don't always have to write a long line of (site:foobar.com OR site:yeehaw.com OR ...) type things in the search box.
Then I got REALLY annoyed by the very small navigation buttons (next page mainly) in Google search. So I wanted to make them BIGGER so that I won't have to point and aim like in CS every time. After I implemented that, I thought that just scrolling with a mouse down should load new results (infinite scrolling) but it should be opt-in feature only. I want to find information fast and don't want to spend time aiming for UI elements.
Then during evenings the bright colors blind me so I implemented built-in dark mode for Google and Bing and created an option to use any custom CSS for search engines.
And the story goes on. So the features that are there were something I personally needed. And since most people need search every day, I figured that why not share the extension with others as well.
Currently there's no business intention and the code is not obfuscated in any way.