ClickCharts is a simple free tool for building flowcharts and UML diagrams.
We really do mean "simple", too, as there are only a few shapes available for each diagram type (12 for flowcharts, 10 for UML, with 9 in a "General" category). But it is at least easy to use.
Click a standard rectangle, say, and it'll appear wherever you click on the page. Choose the "Connector" diagram to drag lines from one box to another. Double-click any shape to enter the relevant text. And as there's no real configurability here (you can't set colours, fonts, line thicknesses, background images or anything else) you'll probably create your chart within two or three minutes.
Once you've finished, charts can be printed, saved in ClickChart's native CCD format, or exported as images in a good range of formats (PNG, GIF, JPG, PSD, TIFF, BMP, WBMP and more). There's no support for working with other charting tools, though. ClickCharts can't import files from other charting software, or export diagrams for completion elsewhere.