iOS: There are countless notes apps available for your iPhone, but Better Notes has a neat little gimmick that makes it worth a look. Instead of dealing with tags or folders for organisation, it allows you to make Twitter-like hashtags that are easy to search through. Better Notes works pretty much exactly as you'd expect. Create a new note, and if you want to link it to other notes, use a hashtag anywhere inside the note. From that hashtag, you can search for other notes, or you can use the built-in search function. It's a tiny, simple thing, but it works really well because all your organisation is in-line so you don't have to remember to do anything. Better Notes also includes lists and image attachments, but the hashtag organisation is really the main draw here.
Better Notes (Free) [iTunes App Store]