iOS: PlainText is a free Dropbox-synchronised text editor for iOS with a paper-like interface, focusing on doing one thing well rather than a laundry list of features.
It's hard to write if you can't focus. PlainText's really straightforward and simple interface removes any unwanted distractions and lets you just write on your iPad, iPhone or iPod touch. Although PlainText doesn't focus on features, there are a few that are worth noting.
Full-screen mode lets you get rid of everything but the page so you can focus.
PlainText syncs with Dropbox so you can edit your text files on any machine. Even if you don't have Dropbox (and you should), it's free for 2GB of storage, and that's more than enough for just text.
The only disadvantage of the Dropbox sync is that I wasn't able to choose the location of the folder even though it looked like I could in the settings panel. Nonetheless, the text files synced up really fast and were on my computer before I could check.
One other notable feature of PlainText is TextExpander support. We're not shy about how much we love text expansion, and it's particularly helpful when you're on a slow-type touchscreen keyboard, so it's a very welcome feature.
PlainText is available for free but has a $6 option to remove ads. I didn't see any ads so I'm not sure that's a worthwhile purchase, but should they show up in the future — or you just want to support the development of a good app — you've got the option.