GraphThing - GraphThing is a tool that allows you to create, manipulate and study 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.
jsflap - A web-based NFA and DFA construction and testing tool.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.