draw.io - Online diagramming application
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
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.
Zoho Backstage - Plan, run, and analyze your event— all from one place
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Meetup for iOS and Android - Discover groups that are all about the things you love