iOS: The iPhone and iPad are great for writing, but if you like to change your fonts around a lot, unfortunately you can't install your own. AnyFont is an app that makes that possible and allows you to install fonts for use in apps like Pages or Keynote.
With AnyFont you can open an OpenType or TrueType font on your iOS device using the "Open In" option in apps like Dropbox. Tap the font you want and AnyFont installs in using a configuration profile. It's slightly fiddly, but does the trick and makes fonts accessible in almost any app that allows you to select a font. It's difficult to tell just how many apps will support it since it's not an "official" way to add fonts, but it certainly worked for me with Pages and Keynote.