Octicons - Your project. GitHub's icons.
draw.io - Online diagramming application
Material Design Iconic Font - Material Design Iconic Font is a full suite of official material design icons and community icons for easy scalable vector graphics on websites or desktop.
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.
WebHostingHub Glyphs - High quality icons that are easy to use with Bootstrap, other CSS frameworks and in any web projects.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.