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.
yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
Online diagramming application
Simplify and communicate complex information using data-linked diagrams.
OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.
Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Prototyping and collaboration for design teams
PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Draw, LibreOffice, ODF, open standards, creative software, vector drawing
FreeMind is a premier free mind-mapping software written in Java.
XMind is a brainstorming and mind mapping application.
Single-user mockup / wireframing / diagramming tool