You start small and scale up, bootstrap from smaller related projects. For example, if I wanted to be the "next Google" I'd work as a consultant on Apache Solr

You start small and scale up, bootstrap from smaller related projects. For example, if I wanted to be the "next Google" I'd work as a consultant on Apache Solr and Apache Nutch, build up capital, then find a search niche that is not well served but potentially lucrative (fields of science and medicine spring to mind), build a usable proof of concept, sell subscriptions at a cut-price rate (whilst still in beta), then look to hire and scale, then look to expand into new markets after the core product is stable. So you see, even with the largest of corporate ambitions, you're still able to bootstrap on next to nothing.