Evernote is one of the most popular note taking apps for virtually every platform. It is available for iPhone, iPad, and Mac and can sync everything seamlessly between them. Take a note or create a grocery list on your Mac and it's instantly available on your iPhone.
Aside from regular typed notes, you can create checklists and many other kinds of documents inside Evernote. Organize your notes inside notebooks, add tags to quickly search for things, and much more. Gather your thoughts neatly and share them with others with Evernote's unique presentation mode. Evernote also supports media uploads including photos, audio notes, and more. There is also great integration with all of Evernote's other apps including Skitch, Penultimate, Evernote Food, and more.
Evernote is also integrated into many other third party services. It's a safe bet for anyone that uses many different platforms or wants a note taking app that's well supported. For instance, if you find an article that you like in your favorite RSS reader, there's a very good possibility that you can send it straight to Evernote. When it comes to settings standards, Evernote is about as well received in the note taking app space as Dropbox is in the cloud storage arena.
Anyone can use Evernote for free on iOS or OS X. You get 60 MB of monthly uploads with no storage limitations. For most folks that are just storing text notes, they'll never hit that limit. You will however see a Go premium reminder inside all of Evernote's apps that you can't hide. Premium users get 1 GB of monthly uploads, offline notebooks, collaboration tools, and much more. Premium memberships also extend to other products offered by Evernote such as Skitch.