They're usually just called essays.
Though in our modern age we often use names related to the platform. So on a blog, they're posts or blog posts. As Zeiss Ikon points out, in a newspaper, they're called opinion pieces. If they're written by newspaper stuff, they're editorials. In a magazine, they're articles.
Chances are though you'll find work by looking for websites and magazines asking for article submissions. If they have any special names for the articles, they'll say so, and then you'll use those names.