If you are a Linux user, and you have ever had to give presentations, you probably used OpenOffice Impress to do it. OpenOffice Impress is a reasonable clone of PowerPoint, though certain things like Animation, work very poorly in OpenOffice, and you can't insert a table (grrrrr....).
Any good presentation has graphics or diagrams in it. There is a drawing tool in OpenOffice to create them, which is a little clumsy, but works. However, the output looks like poo. While OpenOffice will antialias fonts (making them smooth), it won't do so for graphic elements. Thus you end up with reasonably nice looking fonts, and graphics that look like MacPaint circa 1996. What is a budding presenter that wants to stay on Linux to do?
About 2 years ago, I found Inkscape. Inkscape is a vector graphics program that would be comparable to Adobe Illustrator (though it's been 9 years since I last used Illustrator, so I don't claim any ability to compare them). Inkscape is largely designed for graphic artists (which I will not claim to be by any stretch), and has all the sort of drawing tools you would expect for that. I originally stumbled upon it when creating block diagrams in Dia, which while pretty straight forward, again looks like poo if you've got anything that isn't a vertical or horizontal line in it. While Inkscape requires a little more freehanding of shapes, the control over those graphics, and how pretty they look on export, entire makes up for that.
Inkscape's native format is SVG, which is vector based, and thus scalable to any size without looking bad. While it has the disadvantage of being a different tool then OpenOffice it has the distinct advantage that nearly anything drawn in Inkscape looks about 10x better than OpenOffice drawings, without even trying. If you try, you can get stuff that is clearly 100x better.
What I've recently found works best is to create a base image in inkscape with all your elements you are trying to explain. Don't add much text or any arrows showing flow, as OpenOffice does this reasonably. Then use that base image for 1 or more slides in OpenOffice where you draw over it with Arrows, Simple Blocks, and Text. OpenOffice objects can be transparent (or partially transparent), which means doing overlays in OpenOffice works pretty well, and looks pretty reasonable. Believe it or not, the final OpenOffice presentation will look better in Adobe Acrobat as a PDF, which has very good scaling algorithms, and will antialias your OpenOffice graphics objects even when OO doesn't (which, granted, is pretty retarded).
Pictures are worth 1000 words, and good pictures are worth a lot more. But also remember, good pictures take time. Yesterday I spent 3 hours coming up with, and creating 1 graphic which was the basis for 4 slides, then 2 more hours figuring out how to best represent the information on those slides to be clear, simple, and useful. Just like any programming effort, don't think you can crank it out in the hour before it is due.
Ok, back to diagrams and presentations. Happy Friday!