Cytoscape - Cytoscape Official Web Site
draw.io - Online diagramming application
Polinode - Polinode is a tool that makes it easy to perform powerful network analysis.
Nektony Mac Visio Viewer - Nektony Mac Visio Viewer is a fully featured application for Mac OS X operating systems to view MS Visio files.
Gephi - Gephi is an open-source software for visualizing and analyzing large networks graphs.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.