I usually avoid promoting products that I’m paid to work with, but I just love this one: Google Gears. It’s an open-source, cross-browser (IE and Win/Mac/Linux FF for now) plugin that provides a bunch of enhancements for developers of AJAX apps. Most importantly, it lets you add offline support to your AJAX apps without requiring a rewrite or fork. You just replace your existing XMLHttpRequest calls with a wrapper and off you go. The wrapper’s designed to fail gracefully if the plugin’s not available, so one set of code will work for everyone–with or without the plugin, online or offline. Presumably someone will integrate this into Prototype and other JS frameworks shortly, making it easy for thousands of developers to build offline-enabled web apps.
I’ve been watching this develop for the past few months, and I’m really excited about it. I can’t wait to see how people integrate this into Rails and other frameworks.