Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
Xmind - Xmind is a brainstorming and mind mapping application.
DjVuLibre - DjVuLibre: Open Source DjVu library and viewer
MindMeister - Create, share and collaboratively work on mind maps with MindMeister, the leading online mind mapping software. Includes apps for iPhone, iPad and Android.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Coggle - Coggle is a simple, beautiful, powerful way of structuring information.