WriteMonkey is great for working with plain text. It has a few neat features, like being able to set a particular word count goal. It’s geared as a distraction-free writing pad. The idea is to stop fiddling with formatting, and it works. It’s ideal for writing offline.
Windows 7 comes with Sync Center, which is a handy utility which synchronises files on computers on your homegroup whenever they’re online at the same time. This allows you to, for example, use Writemoney to work on the same file on a laptop and a desktop computer. Saves the effort of manually syncing, once you have it all set up.
WordPress itself is useful for organising and navigating your work. You can split a chapter up into bite-size sections just like on Wattpad, and you can also make a page which (with a handy shortcode, courtesy of WP Advanced Insert) pulls all those sections together automatically.
Also worth mentioning is Codepress Admin Columns, which allows you to easily define new columns to be viewable when looking at your post or page list in the dashboard.
If you would like to have Twitter cards for your WordPress site you could also look at the JM Twitter Cards plugin.
I’ve also found GB Search and replace to be useful for mass replacement of words and names within posts e.g. for consistently using a character’s first or last name.
Wattpad is a promising writing platform with plenty of readers looking for a good story and willing to comment on your work.
Sigil is a useful tool for creating EPUB using a little knowledge of CSS and HTML. You may want to do some Google searching to figure out the right way to enter the metadata, but it does have a built in EPUB validator which is nice.
If you can think of some more, just let me know with the Feedback form!