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 Linux. The best alternative is Joplin, which is both free and Open Source. Other great apps like SynNotes are Standard Notes (Freemium, Open Source), Evernote (Freemium), Simplenote (Free, Open Source) and CherryTree (Free, Open Source).
Notational Velocity is a mouseless application for storing and retrieving notes. With Notational Velocity, searching for notes is not a separate action; rather, it is the primary interface. Searching encompasses all notes content and occurs instantly with each key pressed.
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.
Quiver is a notebook built for programmers. It lets you easily mix text, code and Markdown within one note, edit code with an awesome code editor, live preview Markdown and LaTeX cells, and find any note instantly via the full-text search.