HARO (Help A Reporter Out). Free. Journalists need expert sources for their articles. You provide useful quotes. When published, you get a backlink from a news site.
How to Do It
Sign up at HARO or Qwoted as a source. Select your industry. You get daily emails with journalist requests. Reply to ones relevant to your expertise. Keep replies short. Provide a specific, quotable response.
Why It Works
Backlinks from news sites carry enormous SEO weight. Better than directories. Better than most guest posts. And HARO is free.