Pocket Squared Moleskine Notebook
I got one of these in my Maven welcome pack and wondered what exactly I'd do with it. *Talk about a lapse in reason* I'm a writer, a reporter. I have to write stuff down, and this thing fits perfectly (and neatly) into many small purses. Even a good clutch. Added coolness bonus: a very stylish and highly accessorized creative director that I know has one on him at all times.
I have this notebook and I love it. It's tiny so it fits easily in my purse, and allows me a great space to write all of my ideas and plans in. I wouldn't leave the house without it.