The Rhodes framework (available at http://github.com/rhomobile) is an open source Ruby-based platform for building locally executing, device- optimized mobile applications. It is similar in concept to MVC frameworks such as Rails, Merb and Camping but much lighter weight (and hence executable on a mobile device) than any of these. Along the way of course we had to implement Ruby for these device operating systems (iPhone, Windows Mobile and RIM).
In general developer productivity is much higher in Rhodes than writing to diverse native device operating systems and APIs since most of your UI customization can be done in HTML templates (ERB files). Rhodes also provides access to native device capabilities such as GPS and PIM data via an extended set of tags (e.g. <geolocation/>).
These applications are also optimized for interacting with hosted app (SaaS) backends . That is it allows mobile applications to work offline with synced local data by embedding a client for RhoSync (another open source component available from Rhomobile). The Rhodes source tree contains sample apps for SugarCRM and Siebel Field Service and we will be providing a mobile interface to Basecamp soon as well.
Rhodes is initially available for iPhone, Windows Mobile and Research in Motion (Blackberry) smartphones. Support for Symbian and Android devices is under development.
For those of you in the Bay area we'll be presenting at all the local Ruby meetups over the next few months (including the SF Ruby meetup tomorrow night). Hope I get to meet some users of the framework then.