Gephi - Gephi is an open-source software for visualizing and analyzing large networks graphs.
draw.io - Online diagramming application
SocNetV - Social Network Visualizer (SocNetV) is a cross-platform, user-friendly free software application for social network analysis and...
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.
NodeXL - Home
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.