Flat Notes is described as 'The notes app with Google Keep design for Windows 10' and is an app in the Office & Productivity category. There are more than 50 alternatives to Flat Notes for a variety of platforms, including Windows, Android, iPhone, Online / Web-based and Mac. The best alternative is Joplin, which is both free and Open Source. Other great apps like Flat Notes are Standard Notes (Freemium, Open Source), Google Keep (Free), Microsoft OneNote (Freemium) and Evernote (Freemium).
NixNote, formerly Nevernote, is an open source client for Evernote . It is written in Java so it will also run on Windows & OS-X, but the primary focus has been to try and get a usable environment for Linux.