Programs like Microsoft Word and OpenOffice have some drawing capabilities. I like both of them and have been able to do very simple shapes and layouts with them. But sometimes I want to create something more detailed and involved, or my students are looking for a way to create their own pictures and graphics.
I love free stuff, and I have found Inkscape to be a very capable graphics program, and it's free. In fact, it has a lot more capabilities and functions then I will ever be able to use. But, it is relatively easy to use for basic things and has article, tutorials, and documentation to help you.
Inkscape links to another great free graphics resource, the Open Clip Art Library. This site has thousands of images and clip art that are free to use. Most people don't realize that you can't just use any image you find on the internet without crediting it or paying for it. This site makes it easy to find free images. As a side note, Goolge Image Search also has an option for only displaying images that are available for public use.
Check these out and have some fun creating your own images and graphics.