yEd Graph Editor
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Full user review
"A simple to use, yet powerful diagram creation software"
Very flexible in the types of drawing that one can create
The hints that aid alignment and scaling of objects and lines are extremely useful
A very nice and useful grouping feature
Text formatting applies to the whole block, and no quick short cuts for that
PDF output is basically a high resolution image rather than text and vectors
A very useful and simple o use application and for a free software I cannot complain
I had special requirements for a software diagram that I wanted to create and I did not expect any drawing software to enable me to do them, but this one did.