draw.io - Online diagramming application
TestRail - TestRail provides comprehensive test case management for software testing. Organize your testing, boost productivity, get real-time insights, and track progress toward milestones. Integrates with leading issue tracking and test automation tools.
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.
TestLink - Test & requirements management
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Qase - Test case management software for QA and development teams that helps you make your product better.