SynNotes is described as 'Simple syntax highlighted Notes manager with incremental full-text search and GMaill like tags as folders' and is an app in the Development category. There are more than 50 alternatives to SynNotes for a variety of platforms, including Windows, Mac, Android, Online / Web-based and iPhone. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Evernote. Other great apps like SynNotes are Simplenote (Free, Open Source), CherryTree (Free, Open Source), Zim (Free, Open Source) and Zoho Notebook (Free Personal).
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.