Lazarus is a cross-platform IDE for native desktop applications on Windows, Mac and Linux. It produces fast, low-memory, single executable files for easy installation and deployment. Your app can use native OS controls - you won't be gluing together different libraries from different languages like you do with some scripting languages (with all the bloated size and performance issues that entails).
Why isn't it more popular? Because it's written in FreePascal: a modern, object-oriented version of Pascal. A lot of readers will stop right here and never this give project another glance, the thought of using Pascal as simply too outdated or embarrassing.
If you do give it a try, you'll discover one of the best and most overlooked options for native desktop app development. Any downsides? A major one in my view: poor and patchy documentation (which will be a barrier and block for many).