“Scrivener is a powerful content-generation tool for writers that allows you to concentrate on composing and structuring long and difficult documents. While it gives you complete control of the formatting, its focus is on helping you get to the end of that awkward first draft.”
They now have a Linux (beta… but actually runs REALLY well). Used by authors and novelists, it is a pretty cool way to start writing that first book !
Download here (now 32bit and 64bit versions available):