draw.io - Online diagramming application
Serena Dimensions RM - Serena Dimensions RM is a web application to improve the definition and management of requirements across the product lifecycle.
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.
ReqView - Simple and powerful requirements management tool enabling easy requirements gathering, traceability tracking and offline collaboration.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Visure - Visure offers a flexible , modern and complete requirements ALM Platform (Application Lifecycle Management).