My personal tool-kit would be
Writer's Cafe and
CeltX, doing the actual writing in
Open Office. The former is cheap (about £20 last time I looked), but has a pretty feature-rich free version, while the latter is free and open-source, with some paid services available. Best of all, they are both multi-platform; you can install under Windows, MacOS or Linux and they work just as well.
The most important tool I use (ignoring OpenOffice) is probably
Dropbox, a synchronisation service which is very useful for backing up data. I once lost the first 10,000 words of a novel to a hardware issue, so I always make sure I have my important files backed up safely these days. It also has the advantage of synchronising my half-finished stories on my Linux laptop with my Windows desktop PC, so I can write on either one without having to use a USB stick or try to send them over the network.