In plain text, one form is:

  [ ] undone item
  [-] partially done item
  [x] done item

(They're not unuseful.)

[/] I’ve been using this for partials

[-] But I kind of like this too

I use tilde for in-progress items [~]

[*] overdone

That's basically what is known as a tri-state checkbox, or a specific rendering of one.

Cool! I didn't do too much research before building this, was inspired by Adam's book where he mentions this system.