The two main ones I've heard are:

- Airbnb selling Obama/McCain themed cereal.

- Getting people to sign up for your app by literally installing it for them/walking through the sign up process.

EDIT:

Another, IMO, despite hearing that it is a bad idea (I disagree with the idea that this is a bad idea):

- Build it and they will come

The main one is contacting every prospective customer who didn't convert (e.g. abandoned carts).