In addition to giving the world a wealth of cat videos and a new generation of celebrities, the massive video-sharing site YouTube also created a burden for ad agencies – suddenly, marketing executives everywhere were demanding to "go viral."

Google Inc. is promoting the 10th anniversary of YouTube's creation this year, and as part of the celebration, it's looking back at the advertisers that have done so successfully.

For the next nine days, Google is encouraging people to vote on the best ads of the last 10 years.

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The 20 nominees include viral hits such as "The Man Your Man Could Smell Like" for Old Spice, Procter & Gamble's Always brand's "Like a Girl" and, of course, a cat video – produced by Buzzfeed for Friskies.

Advertising is now as much a part of YouTube as anything else: According to Google, four of the top 10 most-watched videos on the site last year were ads.

It has become an exceptionally valuable medium for advertisers, because those that have managed to create something truly compelling have seen that people will sit through advertising content willingly. The most-viewed nominated video, a "selfie shootout" between basketball legend Kobe Bryant and soccer star Lionel Messi, has attracted more than 140-million views for Turkish Airlines.

But the competition also illustrates just how long it took advertisers to figure out how to use YouTube effectively. The oldest nominee – Levi Strauss's "Guys backflip into jeans" – is from 2008.