Pilot Birdie

From 43FoldersWiki

The Birdie is Pilot's super-compact range, designed to fit in organiser and diary pen/pencil loops. The pencils and ballpoints are so thin that they're a bit too small for Filofax loops, but the fountain pen version fits perfectly, and works very well.

They're good solidly made things, nicely designed (if you like brushed steel and simplicity), and very handy. The downsides are the thinnness - you might not want to write for too long with them - and the pencil version isn't pocket safe.