List of nifty tools for drawing diagrams, charts and flow-charts

“Developing web-sites over the last few years, I’ve been stumbling over the same problem over and over again: how can I visualize my idea easily and quickly? Since human being is used to visualize things, trying to understand the idea behind them, recently I’ve spent hours googling for useful tools and tutorials which would help me to create images — mostly, diagrams, charts, chart-flows etc. to visualize my personal ideas to my potential clients.”