If you need to create and share documentation for developers (API, etc), http://readme.io looks like a good choice.
- Very fast and easy web site creation
- Easy content management (articles creation, etc), code formatting, navigation
- API reference is an interesting thing that allow to run requests and see response right away
- Integrated forum (very basic, though, but may be enough)
- Suggestions from documentation users.
- Tables editing and creation is a real pain
- Very strange markdown language
- Themes customization could be easier.
Overall, I can recommend http://readme.io, it looks like the best option so far.
Interestingly, there are just a few options on the market, so I think it is an interesting opportunity to start a new product in this niche. I’d want to have a more integrated product that feeds documents from various places, like GitHub, and aggregates docs in a single usable platform.