Notetaking and blackboard replacement application. Inspired by Xournal. Written in C++/Qt for Linux / Windows. It has the ability to annotate PDF files and has a few other options (e.g. markdown support).
NixNote Alternatives
NixNote is described as 'Formerly Nevernote, is an open source client for Evernote' and is a very popular Note-taking tool in the office & productivity category. There are more than 100 alternatives to NixNote for a variety of platforms, including Windows, Mac, iPhone, Web-based and Linux apps. The best NixNote alternative is Notesnook, which is both free and Open Source. Other great apps like NixNote are Zim, Trilium Notes, TagSpaces and CherryTree.