Google has revolutionized finding celebrity sex tapes and forwarding people to Wikipedia 2 billion times a day.
(It's easy to put a spin on anything)
Groupon does the exact same thing, answer "How do you find small businesses?" The difference is that they compete on price instead of relevancy. Google's value proposition is that they will show you the most relevant ads to your query, so that you don't need to wade through lots of useless shops to get the one you want. Groupon's value proposition is price: the merchants they show you will give you lower prices than the ones you'd find elsewhere.
It remains to be seen whether this is a sustainable value proposition. I can think of two Web 1.0 companies that Groupon reminds me of: Amazon.com and Kozmo.com. But while Amazon became a huge retailing powerhouse, Kozmo deadpooled, hard. The difference was that Amazon's value proposition was convenience and selection, and they used the Internet to deliver both through some pretty impressive logistics management. Kozmo's value proposition was convenience and price, and they used lots of venture capital money to deliver both through selling at a loss. Groupon's business seems to remind me a bit more of Kozmo's though: they compete on price through using direct subsidies.
Groupon drives billions in revenue to local businesses.
There are important differences, and Groupon may fail, but their fundamental market opportunity is easily as big as Google's.