The online Gears developer documentation discusses Version 0.5, released a month ago. It includes nine samples that illustrate the use of the APIs. You can download the source to all of these samples, plus some articles, a couple of tools, and an Apache mod implementing the resumable HTTP request proposal. One of the articles is a tutorial on taking Web applications offline with Gears, written by Omar Kilani of Remember the Milk. Kilani and his team implemented offline functionality for their application in "four caffeine-fueled days."
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Silverlight 2 supports local data caching and isolated local storage, which is one reason to consider it for use as a desktop RIA, although like Google Gears it runs inside a browser. There's also another scenario to consider: With care, you can write a Silverlight Web application that can also be recompiled to be distributed as a WPF desktop application, and the WPF application would stand alone but be able to connect to the Internet as needed. Note that it's much easier to write a Silverlight application and recompile it for WPF than it is to take a WPF application and rewrite the parts that aren't supported by Silverlight.
Silverlight 2 runs as a runtime on Windows XP, Windows Vista, and Mac OS X on Intel-based Macs. Current versions of IE, Firefox, and Safari are officially supported, with "others pending customer feedback"; I have also used Silverlight 2 successfully with current versions of Google Chrome and Opera. The Microsoft development and design tools require Windows; an Eclipse plug-in for Silverlight currently requires Windows but will eventually support other operating systems. The Linux implementation of Silverlight 2 is called Moonlight 2 and is currently in an alpha state.