Cisco Packet Tracer - Download Cisco Packet Tracer, an innovative network simulation and visualization tool used by Cisco Networking Academy students and instructors.
draw.io - Online diagramming application
MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
GNS3 - GNS3 is a graphical network simulator that allows simulation of complex networks.
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.