Planmesh - The easiest way to plan with friends
draw.io - Online diagramming application
tripcake - Doodle for travel plans with friends
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.
Onin - Plan events without leaving the conversation
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.