It's the original form of marketing:

Caveman 1: Hey, Sporg, this redwood branch makes a better club than that oak branch you've been using for the last month.

Caveman 2: Thanks for the tip, Grok. Gotta run -- I need to tell Zog about this.

And thus, word-of-mouth marketing was born.

OK, maybe it didn't happen quite that way, but word-of-mouth marketing (WOM) -- in which one consumer provides product or brand information to another -- has certainly been around for a long time. It's only in the age of TV, blogs, social networks, Twitter feeds and mobile texting that WOM marketing has become something of an art form.

The trick is to give consumers accessing all of these media a reason to talk about a product and promote it themselves. Successful WOM marketing, according to the Word of Mouth Marketing Association (WOMMA), is marketing that "empowers people to share their experiences. It's harnessing the voice of the customer for the good of the brand. And it's acknowledging that the unsatisfied customer is equally powerful."

Now is an especially hot moment for WOM marketing for three key reasons, says John Bell, president of the board of directors of WOMMA. First, people increasingly trust their peers more than they trust advertisers. Second, newly available social media tools mean more and more voices for WOM marketers to target. Third, and perhaps most important, social media tools make WOM measurable.

Take a look at a few examples of campaigns that did a great job of planting the seeds of conversation.