Second Life demonstrated they had a clue a few years ago, with their IP policy on user-created content. I actually think I first heard about Second Life because of a Slashdot article on that policy.
Now Second Life has decided to open source their client and their FAQ about it demonstrates again that they do indeed "get it".
Open sourcing the client, of course means exposing the API (the latter could be done separately, of course, but I don't think it had been until now). This really excites me because of the kind of mashups this will lead to, in both directions.
Very interesting times.
UPDATE: Digging a little deeper, it looks like it's been possible for a while to make XML-RPC calls from within Second Life to the "real world".