Overleaf - The online platform for scientific writing. Overleaf is free: start writing now with one click. No sign-up required. Great on your iPad.
draw.io - Online diagramming application
TeXstudio - TeXstudio is an integrated environment for writing LaTeX documents.
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.
LyX - LyX is a document processor.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.