In terms of integrating your webapp with external webapps I see two different integration types:
1. Inside-Out Integration: in this type external webapps can use services which are offered by your webapp.
2. Outside-In Integration: this integration lets external webapps run in the context of your webapp.
The first integration type (inside-out) is more widespread than the second one (outside-in).
In my opinion if you want your webapp to be successful, you need to think about both integration concepts directly in the beginning of your webapp development. Don't create a webapp without a concept of ecosystem anymore!
What do you think? Do you have experiences and best practices doing especially the second type of integration (outside-in)?