HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
draw.io - Online diagramming application
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
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.
Crucial Human - Crucial Human makes software focusing on human productivity, collaboration, and learning.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.