draw.io - Online diagramming application
Courseroot - Biggest online course database on the web
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.
Alcamy - Free, open self-learning platform. Learn & teach anything.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
CurrencyStack - A free modern currency data API for fintech apps.