The thing, in answering this, is that you're expecting to hear big names such as Facebook. But industry trends are quite different nowadays that they were at the peak of the internet bubble.
- Find a few million
- Spend like crazy on pricey tech
- Hire dozens of people
- Eventually go bust
Now, it's more like:
- Grab an open source solution or a dirt cheap solution
- Custom-tailor it until it works, possibly with a buddy or two
- Set things up in a pay-as-you-grow cloud or in some app store
- Be profitable as soon as you can
- Possibly look for investors if you need them to scale up
And if you read VC blogs for any extended duration, you'll quickly conclude that step 5 really is optional. It completely freaks them out. Many times you don't need VCs to scale at all (e.g. a popular iPhone app). When you do and your start-up is profitable, you can get capital at a low cost. They're getting thumb-screwed out of business.
Anyway, Groupon comes to mind as something that was originally built on top of WordPress, but eventually sought capital. Facebook originally was a quick hack in so far as I understand (I'd be happily wrong if it wasn't), but they sought capital too.
Better examples will likely come from Apple's AppStore: I take it that Chillingo (Angry Birds) or PixelMator are both good examples. And there are many 1 or 2 person joints in there who make a comfortable living selling a few apps.