Mac users (you weirdos!) have the option of Write Room, an alternative to MS Word that gets the computer out-of-the-way. It creates a full screen environment that lets you focus on your writing. Dark Room is the Windows clone.
For very large projects, you might want to use Scrivener. Scrivener is bisexual (i.e. is compatible with both Mac and Windows environments).
If you don’t want to use software you have to download or install, you could also try the web-based app Writer. Writer claims that it is used by over half a million writers. All you have to do is create a free account and get writing. A ‘pro’ version (that costs money) is available.