I recently got going with ReadMe for a side project* for documenting my API and can attest that both Greg and Gabe have been extremely responsive at fielding my bugs and feature requests.
The ReadMe product is very well polished and the support of Markdown means writing documentation is easy without a complicated editor layer to learn. There's also a palette of pre-configured widgets that you can drop into your pages to handle embedding code, images, custom HTML/CSS, tables, etc. if you want to get fancy.
Nearly every time I log in there's a new feature, bug fix or enhancement - it's great to see the guys build such a useful tool so quickly but without impacting quality or uptime.
* ReadMe powers http://dev.avisohq.com (warning: work-in-progress)
