Off the top of my head, I think plenty of service businesses start out this way. E.g., consulting companies, legal firms, cleaning companies, hair salons, etc. Sometimes they start accidentally: a person does a friend's tax return or a little legal work on the side from their day job. Eventually they build up enough of a reputation and a client base that they say, "Now let's make it official."

I think the same trick can be pulled off with food. A good SF example is the Creme Brulee Cart:

http://www.thecremebruleecart.com/story/

It started out with one tiny handmade cart, grew up to a food truck, and eventually added a physical store.

For startups that are service businesses, the Concierge MVP is a great example of ways to go after early revenue. For example, consider a friend's startup:

http://amazingairfare.net/

This is the kind of thing that at scale would require sophisticated software. But for a small number of people, you can just do it manually.

Another good concierge MVP example comes from my colleagues at Code for America:

https://www.youtube.com/watch?v=lqTFi2U2Ebc

They aren't getting revenue here, as it's a public service, but you can see how the manual-to-automated approach can work incredibly well.

If you're near to one, consider dropping by a Bootstrappers' Breakfast:

http://www.bootstrappersbreakfast.com/