The D1 pen refill is a standard pattern used in most multi-function pens and many compact or expanding pens. Made of metal, usually with a hollow end, it measures 67mm in length and approx 2.3mm in diameter.
We have D1 refills from Lamy, Schneider, Zebra, Tombow, Faber-Castell and others, and thanks to the huge variety of pen types that use them there is a good choice of ink colours including the usual black, blue, red and green. Due to the small ink capacity, most of the refills here are fine point, though there are medium and even broad tips too. In addition to ballpoint refills there are highlighter refills (in actuality a bright ballpoint pen, as normal highlighter tips can't be accommodated in this format), and stylus tips. If you have a multi-function pen you can normally swap any of the functions for alternative refills - eg. change that little-used red ballpoint to a green refill, an extra-fine black, or an orange highlighter.
Worthy of note is the Tombow BR-VMP refill which is pressurised - meaning it can write upside down, in cold conditions, or on wet paper, just like a Space Pen.
If you have a refill that looks like one of these but with a plastic screw-top, then it's usually a D1 you need - the screw-top is part of the pen, not part of the refill. Pull the metal refill out of the screw-top!