For example, cleaning companies - often started by one person doing the cleaning - are an absolutely textbook example of entrepreneurship.

Some of those entrepreneurs then become millionaires :)

(Updating this post: obviously a lot of unpleasant work is done by non-entrepreneurs. But the maxim of "find something necessary but unpleasant and start a company doing it as a high-success-chance play" is also very true: entrepreneurs don't, as a class, all avoid the nasty stuff.

In the UK, the phrase I'm looking for is "where there's muck, there's brass.")