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. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Google Keep. Other great apps like Flat Notes are Microsoft OneNote (Freemium), Evernote (Freemium), CherryTree (Free, Open Source) and NoteLedge (Paid).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.
Notezilla is the leading sticky notes app for Windows, Android, iPhone/iPad and the web. Set reminders to sticky notes, Pin sticky notes, attach them to websites, docs & folders, sync between PCs and other devices, organize using folders and tags.