HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
draw.io - Online diagramming application
Rivers IM - Simple, fast & secure team collaboration 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.
P2 - Share, discuss, and collab with your team. By Wordpress.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.