This is the only pen I have owned that is suitable for writing technical notes. It has a .5mm point, which allows me to write small details such as subscripts and fractions. It is a ballpoint so it doesn’t bleed which is an annoyance I’ve encountered when writing small details with other pens. It doesn’t gum up as my Zebra ballpoints tended to do and so is suitable for drawing diagrams. It can write continuously without drying up and flows well over the paper. Most .5mm pens I’ve used dry up after writing for a while and are so sharp that they end up scratching the paper.
It’s also affordable. Unfortunately, while it only costs $1 a pen there is only one supplier in the US (that I’m aware of) and you have to buy twelve pens at a time.
You can buy it in black, blue, red, purple, or green with a .5mm point. You can also get it in black with a .7mm point.