draw.io - Online diagramming application
Uxcel - The easiest way to learn UX/UI design
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.
UX Challenges - Done reading about UX? Start doing it.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Weekly UX Exercise - Receive challenges top companies use to interview designers