I had a Fisher space pen once, years ago, and hated it - it was always blooping out ink and you had to wipe the end off before using it, or it would streak. I finally lost mine and never replaced it.
I bought a couple of Zebra telescoping pens and found them very useful. Easy to extend, and when collapsed fit well in a pocket or small pouch, or next to a Leatherman tool. No key chain loop, though.
I picked up a Nite Ize "Inka" a while ago and its utility has grown on me, though it is seriously flawed. It's a keychain format pen, I can pull it out of its little carrying tube and use it as a short pen, handy for quick notes. If I need to write something longer, I can theoretically unscrew the storage tube from the key ring cap, unscrew the cap on the pen, turn the handle around and screw it into the pen body, then screw the pen cap onto the storage tube, making a regular-length pen. To store the pen again, merely reverse all these steps being careful not to drop any of the parts. It's no worse than field stripping an AR 15 while standing on a ladder in a rainstorm. But the stubby pen works well enough for the short bursts of writing I use it for. I put the key ring though the coil binding on a mini notebook and it rides in a cargo pants pocket, convenient for travel notes. They are expensive for what you get.
It's a stupid hack, but before I started carrying the Inka, I just carried a short metal refill for a Zebra pen in my wallet - stow it next to the center fold and it doesn't get bent, and I haven't had one leak (yet). It's the ultralight version of a pen. Uncomfortable to write with for any length of time, but OK for endorsing the back of checks or scribbling down items for a grocery list.