While there are a lot of different answers to that question, it's probably safest to say that link building is evolving. Certainly the lawless days of buying links, spinning unintelligible articles, and spamming endless blogs with bogus comments are over.
Today at Mindscape, we believe the most valuable link building is a byproduct of creating great content. There are several other effective link building methods, but the benefits of great content extend far beyond its value for link building. Great content should be the backbone of your online existence. Creating great content gives your visitors the information, the online experience, the value, and the great content that they want. Not only will you gain more clients/customers - these visitors can then become brand ambassadors for your products/services, and help spread the word for you.
Great content is shareable. Your goal should be to create content that people can't wait to share on their Facebook, Twitter, Pinterest, forum, or blog page. The more people who share your content, the more people who are exposed to your great content.
Of course just creating great content isn't enough - you will need to make sure your content is optimized and findable for search engines, and then do the legwork of finding appropriate online outlets to post your content.
So, great content + great distribution = MORE CLIENTS/CUSTOMERS!