draw.io - Online diagramming application
Lampyre - Lampyre - an efficient data analysis and OSINT multi-tool for everyone.
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.
Linkurious - Search and visualize your graph data through a simple web-based interface
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Maltego - Maltego is an open source intelligence and forensics application.