Barely over a week ago, Google unleashed Google Gears, an API and browser extension that help Web developers make their online applications work offline, too--so you can use 'em even when you're disconnected.
Google's proof-of-concept implementation of Gears was with its Google Reader feed reader--which can now download RSS feeds for offline browsing. It does what it says it does, and does it well. But it also serves as a good example of how truly Webby apps may not work very well offline: Any RSS feed that includes just a snippet of text, with a link back to the originating site, is useless when you aren't able to connect to that site. (And that's true of a lot of feeds these days, including ours.)
Yesterday, I happened to visit Remember the Milk--an extremely nifty browser-based to-do list--and saw that it had a new offline mode, implemented with Gears. (The clever Australians behind Gears work fast!) It works well, too. It doesn't replicate all of RTM's features--for instance, you can't add contacts--but you can add, delete, and organize tasks, exactly as you can when online. Once you're connected again, it synchronizes any changes you've made back to the server. Accomplishing useful work in my browser when I was disconnected--I'd intentionally removed my Ethernet cable and shut off Wi-Fi in order to test offline RTM--was kind of a thrill, actually. I felt like I'd just witnessed a sea change in what a Web browser is capable of doing.
Remember the Milk's offline mode has whetted my appetite for more offline stuff in online apps. Thanks to Gears, I suspect we'll see a lot of it--surely Google itself is working furiously on offline capabilities for Gmail and Google Docs and Spreadsheets. If anyone's compiling a definitive list of services that are using Gears, though, I haven't found it--the official Gears blog hasn't been updated this month...