Ask HN: Boring businesses which make non-boring money?

OK HN crowd, I need some help brainstorming. I would love to hear from you about businesses you have experiences with that are extremely profitable but are totally boring and non-sexy. Say, for example, scaffolding rental. Let's focus on businesses with some scale, things that you've seen get to $10-30M+ in revenue and $3-5M in profit. Recurring revenue a plus.