draw.io - Online diagramming application
i3 - A dynamic tiling window manager designed for X11, inspired by wmii, and written in C.
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.
dwm - dwm is a dynamic window manager for X. It manages windows in tiled, monocle and floating layouts. All of the layouts can be applied dynamically, optimising the environment for the application in use and the task performed.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Openbox - Openbox is a highly configurable, next generation window manager with extensive standards support.