Jama - Jama combines requirement management software with enterprise collaboration capabilities to solve product delivery problems.
draw.io - Online diagramming application
Reqtify - Reqtify is an easy to use, interactive application for managing requirements.
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.