Nature is full of delicious abstract forms, many of them powered by the controlled chaos of fractal logic. Concept artists in particular need to be able to freely invent and resolve abstract forms.
H. R. Giger, John Berkey (above), Moebius, and Syd Mead are a few artists who create fascinating abstract forms without sacrificing their representational power. 
For those who work with digital tools, there’s a open source application where the computer partners with the artist at the level of random shape-generation. (Above: Andrew Jones).
Seeing those computer-generated shapes then stimulates you to come up with more ideas. (Above: Nicolas Francoeur) It's like mutation and natural selection. You can rationalize them in any direction you want. Or just leave them and enjoy them in all their inchoate glory. The open-sourced Java-based app is called Alchemy.
Ten years from now, I'm sure software like this will do a lot more than just 2-D shape generation. Imagine a random form generator using a deeper database including anatomy, perspective, nature-based texture logic, and interactive lighting.
A tool like this would become a fundamental ally of concept artists. You'd start with a lump of digital clay, set some parameters---gravitation, climate, habitat---and the lump would evolve before your eyes (with a little guidance, of course) into whatever sort of creature, architecture, or vehicle you're trying to come up with.
For those of us who still work with brushes and pencils and paint, there’s always the sponge, straw-and-ink, toothbrush splatter, crumpled paper, tea leaves, wood grain, Rorschach ink blots, and erratic hand-movements.
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Alchemy website and videos.
Thanks, Ben Schram, who told me about this. Visit Ben's website and see some of his Alchemy-inspired sketches.