Gobbling Google: The search giant's biggest acquisitions (pictures)

Google's purchase of the smart home company Nest (and its smart thermostat, pictured above), generated a lot of buzz on January 13, 2014. The tech titan has acquired more than 100 companies over the years. Not all of them were quite as expensive as Google's latest purchase, which cost the company $3.2 billion, but that doesn't mean the rewards weren't huge. We take a look back at some of Google's biggest purchases.

This was a definite win for Google. Once an under-the-radar startup in Palo Alto, Calif., Android is now the namesake of the world's most popular mobile operating system. Google quietly purchased the then little-known company in 2005 for an estimated $50 million. Five years later the operating system was on 33 percent of smartphones, prompting Google to call Android the "best deal ever." That rings even more true today; the Android operating system powers more than 80 percent of smartphones worldwide.

Google spent $1.65 billion for video-sharing service YouTube in 2006, just as the service was picking up steam. The company was already a hot item. Google reportedly beat out the likes of Microsoft, Yahoo, and News Corp. to get YouTube, which at the time hosted 100 million videos and drew 20 million unique visitors a month. These figures pale in comparison to YouTube's numbers today: 1 billion unique monthly visitors now watch more than 6 billion hours of videos every month. The service has even fulfilled Google co-founders Sergey Brin's earlier vision as an aggressive advertising platform.

Google's largest purchase to date, Motorola Mobility also came with plenty of drama. Google announced in August 2011 that it would purchase the struggling handset maker for $12.5 billion.

Motorola gave Google the resources to get into smartphone manufacturing while also delivering valuable patents for wireless technologies. Those same patents were a lightning rod for patent lawsuits, and a recent ruling went in favor of Google's rival, Apple.

It has not been an easy path for the Motorola unit, though last October, Google unveiled the first phone to come out of Motorola under its wing, the Moto X.

Update January 29 at 2:42 p.m. PT: As January 2014 wound down, Google announced that it is selling the Motorola unit to Lenovo for just under $3 billion.

Google paid $3.1 billion for DoubleClick in 2007, before laying off a fourth of the advertising company's staff. The acquisition is one of Google's more expensive purchases, aside from Motorola Mobility and, now, Nest. Google wanted DoubleClick to beef up its display advertising chops, helping with its targeted ads strategy. The service, like other advertising services, tracks user activity online in an attempt to figure out which ads are more relevant to the viewer. Targeted advertising is often controversial among users, given the amount of data these services collect.

Mapping social media app Waze got plenty of attention in 2013 as several of the big tech companies, including Facebook, reportedly tussled for it. Google ended up with the winning bid of $966 million. The mapping service, which let users choose cute avatars to identify themselves on the service, relies on its users to report helpful driving tips, like road conditions, traffic accidents, and where cops are sitting in wait. Waze found itself a rising star after the disastrous launch of Apple's mapping service. After the purchase, Google promptly integrated Waze's data into its mapping services.

This recent purchase -- along with the acquisition of a slew of similar companies around the same time -- show Google's commitment to robotics. Google bought Boston Dynamics in December 2013 for an unknown sum. The robotics company rose to Internet fame for its animal-inspired robots, including Cheetah, BigDog, WildCat. The company's YouTube videos show these mainly gas-powered robots leaping through ice, snow and other obstacles. Many of Boston Dynamic's creations were funded by the U.S. Department of Defense's Defense Advanced Research Projects Agency.

Google acquired Zagat in 2011, paying $151 million to purchase the restaurant-rating company. The deal, which added reviews of establishments in more than 100 cities, was meant to boost Google's local business listings. Google integrated Zagat's reviews into its widely-used Google Maps service, so when people are searching for locations, they can see businesses as well. The company recently overhauled the service, updating the site, introducing new iOS and Android apps, and making it free to access the directory. Unfortunately for Google, it still hasn't been able to compete with Yelp, which is at the moment is conquering this market.

Similar to DoubleClick, the purchase of AdMob in 2009 was meant to bolster Google's advertising offerings. Google paid $750 million for AdMob, which specialized in mobile advertising, or more specifically, placing ads on Android phones. The service has helped Google grow its mobile advertising dollars -- an increasingly important task now that users have moved onto mobile devices for their everyday needs.