Ranked in these QuestionsQuestion Ranking
Pro Tons of icon sets
Has flowcharts, uml, chemical engineering, electronics, and dozens of other complete sets, built in.
Pro Free and open source
Pro Custom gzipped XML format
Dia has not been updated since 2009. It's already a very complete piece of software, but still sad.
Con UX is a joke
UX is not good enough. For example, to edit element text you must double-click it, then click text box, and then you can start typing text.