Mountain Desk is a (third-party built) Basecamp desktop application for Windows and Mac.
So how does Mountain Desk work? Well its an application developed in Flex and deployed via the Adobe AIR platform. Once you launch the application and sign in to your account (with support for multiple accounts), the application immediately starts to retrieve your data from Basecamp. You can now work just like you would normally using Basecamp while all your changes automatically stay in sync. Furthermore unlike regular Basecamp, you can continue to work even when there is no internet connectivity. So if you are on the road, travelling by air or on a client site, the application continues to function normally and allows you to make changes and additions to your project data. As soon as mountain desk detects internet connectivity all your changes are automatically synced.
Mountain Desk also has a mobile (tablet edition) in the works that will be released soon.