Startup meme mentions quite a few efforts:
Less than a month ago Adobe launched Apollo, enabling developers to create web applications that work seamlessly irrespective of whether a user is connected to Internet or not. Apollo was soon challenged by Dekoh, which was pretty much offering the same functionality with the added glitters of being open source and cross platform as a result of being written in Java. This was followed by Slingshot, a Rails framework developed by Joyent, that brought offline functionality to Rails apps.
Today Dojo has released their Dojo offline toolkit. Dojo Offline is totally free, 100% open source and consists of a javascript library and a 300k cross platform, cross browser download that helps to cache the user interface of your web application for offline use.