I'm committed to text-only information. No matter how many notes I'm writing, I can always use grep or write my own programs to find what I want.
If you're on a Mac, The Archive is great (https://zettelkasten.de/the-archive/). It's what I'm using
zettelkasten.de has lots of good reading (thanks, Sascha and Christian!), but look to the other sources I've listed to get a more thorough idea of what a zettelkasten is (I call mine a thinking-box) and how to use one.