Which European product or service do you think is underrated and should be promoted more?

Posteo email (Germany).

They're one of the cheapest privacy focused email providers at 1 euro a month, offer very cheap plan upgrades based on what you actually use/need, run off 100% renewable energy, and have a really nice simple and intuitive interface.

Tuta is probably a bit more private overall, but if you only use email for mundane stuff and just want to prevent my data from being sold to advertisers, posteo fulfills that need with minimal expense very well.