dradis - Dradis is the open-source reporting and collaboration tool for IT security professionals.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Lampyre - Lampyre - an efficient data analysis and OSINT multi-tool for everyone.
draw.io - Online diagramming application
Netglub - Netglub, the really open source information gathering solution.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.