iOS (Jailbroken) Whited00r is the easiest way to get the newest version of iOS on an unsupported phone and it has been updated with a bunch of iOS 7 features. It's essentially a very complicated theme and it now lets you get iOS 7 on older devices like the iPhone 2G, 3G, and more.
iOS: Pad & Quill is best known for their phone and tablet cases, but they just took the wraps off of a good-looking note-taking app for the iPhone and iPad. It supports multiple notebooks, photos and graphics in notes, automatic table of contents, and iCloud syncing so you have your notes on all your devices.
Android: There are a good number of office suites for Android. None of them can outright replace the office software on your desktop, though. AndrOpen doesn't need to, as it's a full port of OpenOffice.
iPhone: Mind-mapping is an effective technique to take notes of your thoughts, but it can be a little difficult on the limited screen space of an iPhone. Mindly makes mind maps beautiful and easy on the small screen.