Among startups we've funded, I know at least Greplin, Optimizely, Appjet, and ReMail have/had founders who previously worked at Google. I'm sure there are at least that many more, but I don't remember where most of the people we funded worked before.
I guess Biz Stone came directly from Google to found Twitter.
Given Google's size and the quality of the people who go in there, I'd have expected dozens if not hundreds of companies to be spawned by ex-Googlers.
I often wonder if certain companies simply produce more founders because of culture, money or simply bias in who they hire. PayPal is the classic example of a company that pumped out a good number of successful founders, but I can't identify any particular characteristic of that particular company that might have caused it.
I'd love to see some analytics about where the most successful or prolific founders come from.
Google may have very good technical people, i.e. they are capable, but I am not sure how they would fare as "leaders", "influencers", etc... it's also a company culture thing. If your company does not promote that kind of behavior, it is very unlikely you will see people stepping out to try things on their own. And if Google is successful at making their employees happy in some way or another, that also puts the barrier of leaving Google to a much higher point.
I've known plenty of really sharp, talented developers that have zero interest in ever even looking at a P&L sheet.
When you've immersed yourself in a certain sub-culture (say startups), it's sometimes easy to forget that not everyone out there shares that model of thinking.