There are a couple I have in mind. Like many techies, I am a huge fan of RSS for content distribution and XMPP for federated communication.
The really niche one I like is S-expressions as a data format and configuration in place of json, yaml, toml, etc.
I am a big fan of Plaintext formats, although I wish markdown had a few more features like tables.
Because non-open ones are not available, even for a price. Unless you buy something bigger than the “standard” itself of course, like a company that is responsible for it or having access to it.
There is also the process of standardization itself, with committees, working groups, public proposals, …etc involved.
Anyway, we can’t backtrack on calling ISO standards and their likes “open” on the global level, hence my suggestion to use more precise language (“publicly available and sharable”) when talking about truly open standards.