Google Gears can no longer be downloaded.
Google Gears was one of Google's first forays into the Web 2.0 world. It allowed users to download RSS feeds in applications such as Google Reader to their hard drive for reading later.
Now Google has extended Gears to Gmail, Google Calendar, Google Docs, and just about any application that's been developed in Ajax. It even works with Picasa on mobile devices. Google Gears also been recently updated to work with Firefox 3.5 after several compatibility issues.
Once installed, Google Gears prompts you whether you want to sync the application you're viewing with it. Synchronization doesn't take long and shouldn't slow down your browsing experience. If you're happy to trust Google Gears syncing to your apps, then just click 'Trust Google Gears' and the operation will take place automatically every time you use the site. You can configure which sites are allowed to use Gears by simply clicking on 'Tools' in Firefox and selecting 'Gears Settings'.
Once synced, you can disconnect from the Internet and use your Gmail, Picasa Albums, Calendar etc as if you were online. This is great for tidying up and organizing what you want to do on the train, in the airport or anywhere you don't have access to the Net.
Google Gears is a very basic but handy way of using your applications offline. It's just a shame that it still doesn't support more applications.