Let's get the famous story out of the way first. Everyone 'knows' that ballpoints didn't work in space, without gravity, so the Americans spent millions of dollars to develop a special pressurised ballpoint that did work in space. The Russians just used a pencil. Well, not really.
The Americans and Russians both used pencils. Paul Fisher, an inventer and pen maker, developed the pressurised refill, creating the 'Space Pen'. Then both the Americans and the Russians bought them from him and used them in space. Graphite is conductive, so snapped off bits of lead were considered a bit of a safety hazard floating around in a space capsule, so they were glad of a good alternative.
Oh, and standard ballpoint pens do work, and astronauts do sometimes use them, they're just not as reliable in zero gravity and extreme temperatures.
But Paul Fisher's Space Pen was reliable in these conditions, which makes it useful in all sorts of conditions, not just in space. They'll write better on damp paper, and greasy or dusty paper. They'll cope better in freezing conditions or excessively warm conditions. They have a much longer shelf life than standard ballpoints, too. They're just more reliable here on Earth, as well as in space.
The classic Bullet Pen is a wonderfully simple and practical design. Designed in 1948, it actually dates back to almost 20 years before the Space Pen existed. The Backpacker is a similar design, but modified for, well, backpackers, but useful for all sorts of other situations. It's aluminium rather than brass, making it lightweight. It has a keyring attachment on the end of the cap, for easy attachment - clip it somewhere you can get to easily. A more shaped grip section makes it more usable when wearing gloves, and the cap snaps firmly on, so it won't slip out when carrying it hanging from the ring.
Measures just 100mm long when capped, but extends to 133mm when posted for writing. 10mm wide, 8mm at grip. Weighs just 16g.