Like to keep a pen handy but don't want to look like you're carrying an office in your bulging pockets? The Ohto Petit-B compact wallet pen fits easily in your pocket — and even in the fold of a wallet.
The pen measures a meagre 3.1 inches tall when it's closed, and when you're using it with the cap placed on the top, it expands to a comfortable 5.1 inches. While it may not be quite as space efficient as the Moleskine + flat pen for a back-pocket writing kit, it does fit nicely in the fold of your wallet (as verified by the folks at Cool Tools), and is certainly more comfortable when you start writing with it.
The Ohto Petit-B will set you back $US8 from JetPens.