If you're writing opinion essays (and want to be publishing them), you might want to check out this Op-Ed Resource from the DeWitt Wallace Center at Duke University. The good news is that it offers plenty of tips for writing a good piece (and representative op-eds with explanations of why they're effective). And you'll also find plenty of information about newspapers, magazines, and websites that publish op-eds.
The not-so-good news is that the site doesn't seem to have been updated very recently, so you'll definitely need to double-check (and maybe "Google" to find new links for) the market information provided.
(Of course, if you're seeking potential homes for your essays you can also consult our own Directory of Paying Essay Markets.)