Lucidchart is a web-based application to draw diagrams.

LibreOffice is a full-featured office productivity suite that provides a near drop-in replacement for Microsoft Office.

yEd is a free general-purpose diagramming program with a multi-document interface.

Calligra Flow (Kivio) is free diagramming software that is a part of Calligra Suite, an integrated graphic art and office suite by KDE.