From Jared Hunt
Evernote is a free service that I’ve used on Mac, iPhone, and iPad. There are versions for other mobiles and for Windows as well. It is a simple word processing program that incorporates online storage so you can share your notes across any number of your computers and devices.
One of the problems I’ve always had as a GM is keeping my notes organized. I’m one of those people who likes to have a way to record good ideas no matter where I am and, in the past, that meant lots of paper scraps, napkins with notes scrawled on them, and journals. Trying to find a specific idea was quite the chore.
Now I just use Evernote. No matter where I am, I’ll almost always have access to either my phone, my iPad or a computer with internet access. As long as I have at least one of those things, I can easily record my ideas and they are instantly synced across all my devices.
I also appreciate the ability to create and store notes via text, picture, or audio. If it’s useful, you can also add a geotag to sort by location.
Evernote has a robust system for tagging your notes that makes searching and sorting easy. I have a tag for each campaign, one for NPCs, world info, magic, and so on. You can add as many tags to an idea as you want.
I haven’t tried it yet, but I suspect it could be a great way to take notes in-game too.