1. Writing quality articles - I would expect you can build a clientele and even employ more authors as you become knows as a good author.
2. Video creation services - creating a method to efficiently create videos that help people explain their sites or services could be the type of thing where you can get bigger over time and refine your process.
3. Software products - these days it is not that difficult to hire a programmer and make products people can use.
4. Building a list of subscribers over time, generally as a result of providing great content, software or services.
I'm sure there quite a few more examples of WSOs that can work for a long time, and I would love to hear more.
I won't comment on the unsustainable WSOs, as I think Matt Cutts does a great job of explaining that aspect here