I'm not a big writer myself (sometimes an edible poet) but I did learn of Storybook, which is an excellent tool for building characters, locations and keeping track of plot-lines. As a bonus, it's open source, and we all love open source software right!
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Another application that I haven't tried but seems like it would be a good writing tool is FocusWriter, which is another open source application that aims to be a "distraction-free word processor designed to immerse you as much as possible in your work."
One final tool that isn't specifically for writing but could be useful none-the-less is the mind-mapping tool XMind. I love the concept of mind mapping, it's an excellent way to collect your thoughts and XMind is one of the more intuitive applications I've found. It is also open source, although they do have a "enterprise" edition. However, I think the regular edition is perfectly adequate for home use (I'm not really sure what the difference is to be honest). I've tried Freemind, but somehow it seems a little less polished and a little more clumsy compared with XMind.
So what tools do you use if any?