Personally I use OneNote to just write things (since it syncs between all my devices, both mobile and computers) and then OpenOffice to do the editing afterwards, but other really useful programs are:
Wordweb is my go-to thesaurus (every writer's favourite dinosaur). It can sometimes be a bit fidgety when you're using the hotkeys to look up words, but it's free and can be run from a flash drive.
The Everchanging Book of Names is quite useful when you need to come up with names that at least sound like they're based on a different language. The program is shareware and has a somewhat limited functionality. (You can only generate five names before you have to reload the language you're using. This only takes a second so it's not that much of a hassle.)