Don't let the simplicity of use fool you. Both Kivio and Dia, two free software diagramming tools, are very efficient at what they do. If you need to design a complex flow chart or create a no-fuss UML diagram then you could do a lot worse than to choose either of these packages. The tools have 90% of the expected functionality with only 10% of the hassle and fuss that more complex and unnecessarily feature rich proprietary diagramming tools deliver. The learning curve is small and the end result is potentially professional.