I wrote a tutorial on abstract designs in Inkscape, such as used for wallpapers, website banners and sigs.
Tutorial Series: Abstract Wallpaper
As Inkscape had many cool features for creating abstract designs without effort, I figured that just one tutorial would only show the tip of the iceberg, and that a series of tutorials would probably better. So I made a few mockups and made a first selection of techniques to be covered.
The first tutorial is published and features an abstract design of Australia in cool blues. Right now I am working on a warm hearted tutorial in red for Valentines. I expect the tutorial to available tomorrow, after some further testing by my Inkscape guinea pig.