RhoMobile (http://rhomobile.com/) offers an open source framework which allows Ruby developers to write mobile apps using web technologies.
I have been using it for about a year now. You can find out the details of it on the website linked above, but I just wanted to go over some of the issues I have found and cover some of the pluses in minuses from a personal perspective to help you decide whether or not this is a technology you will want to exploit on your next project.
So what is it?
At a basic level, a Rhodes (http://rhomobile.com/products/rhodes/) app comprises of a small Ruby web server/framework and an embedded browser. Server is a little bit of a misnomer because it lives on the client, but essentially it's like a small Rails server which is only used by one client (though in theory you can connect to it from another client if you so choose).
So why use Rhodes instead of creating a mobile web app?
Well, for one, you can package a Rhodes app and sell it in the app store, android market or app world.
It also (if you need it to) can sync with a sophisticated sync server known as RhoSync (http://rhomobile.com/products/rhosync/). As of the time of writing, RhoSync is built upon Sinatra and Redis (it used to be Rails and MySQL). This means that a user can manage and manipulate his/her data on their phone without the need for a constant internet connection and that data can be periodically synced with the back end. RhoSync manages all the potential data conflicts and only syncs the differences. It can connect to practically any kind of data store (databases, REST APIs, etc...).
So what are some of the limitations of Rhodes?
It also really is targeted at the high end smart phones. It is quite well optimised for iOS, but on slower Android devices (anything with less than a 600Mhz processor) it can be quite slow and sluggish.
Another thing to get one's head around is that even though the framework is Rails-like, it is not exactly the same as Rails (it is quite stripped down). The RhoMobile team have been busy adding features however so its ecosystem is growing. When I started using it, there were no models (only controllers and views). Since then, Model support has been added as well as MSpec for testing.
Once your product is done, adding it to the respective app markets is also quite a hassle. This is by no means the fault of RhoMobile or the Rhodes team, but it is something to consider when trying to decide between doing a web based app vs an installable one.
The other limitation is because Rhodes is designed to work with most of the major smart phones on the market, you haven't got access to all the available features on a specific mobile phone. There is limited camera and calendar support, but currently no audio or video recording (it's in the road map, but not yet available). So you are not going to be able to write an app like Shazam or Google Maps using the framework...
If you like Ruby, web technologies AND mobile apps then it's definitely worth taking a look at RhoMobile. It's especially suited for Enterprise App development where you may not need the latest fancy UI enhancements, but you do want to deploy on all the platforms that your team is using with a single code base.
You can find more information on the app I developed at