One tool I've made a lot of use of is Dia, a freeware program that draws very cool diagrams. It can be used for flowcharts, concept diagrams, and I even used it for a "rich picture" for one assignment.
One of the things I love about it is that if you are drawing connective arrows that are anchored to two or more boxes, and move the boxes, the arrows come with them. You can import images, and draw just about any shape with the inclusion of an add-on, also available for free. You can make the boxes any colour you want, which makes drawings very visually appealing.
Drawings can be saved in the native format, or exported to either jpg or png format so that they can be used in other programs.
If I were to find fault with it, it would be that entering and editing text in the boxes is a bit awkward, sometimes a bit hit and miss. It would be nice to be able to specify default font type, alignment, font size without having to change it to what you want for every box.
It's available for download at
http://live.gnome.org/Dia
My rating: 3.5 starts out of 5!
