Windows/Mac/Linux (Adobe AIR): GMDesk is a single-window frame that can hot-switch between Gmail, Google Calendar, Google Docs, and other G-apps, but it could use a few tweaks to make it truly convenient.
The whole idea of apps like GMDesk is to get your web-based productivity tools onto your desktop, without the need for a browser, as with Mozilla's Prism, which we've explained in terms of distraction-free webapps. GMDesk does put all of the majorly helpful Google apps into a single window shell, lets you switch between them hotkeys (Ctrl+1 for Gmail, Ctrl+2 for Calendar, etc.), and switch to domain-hosted Google Apps versions of those webapps instead, which alone might make GMDesk worth it for anyone working both a personal and work-related Gmail account.
But GMDesk, last updated in June, lacks for customisation options. You can't, as far as I can tell, change the hotkey switches. More frustrating, though, is the inability to change the font and text sizes on your apps. Google Calendar and Reader start off in decent sizes, but most serious users are going to get frustrated with Gmail and Docs pretty quickly, unless they're using their system's built-in magnification tools, and then that's kind of a pain. Maps has its own zooming capabilities, so that's pretty much a wash, and Picasa Web Albums kinds of sticks out for not being an everyday-use tool.
Still, if you're a Google app fiend and would like to keep them separated from your main browser window, GMDesk might be a step up. It's a free download, requires the Adobe AIR platform to run.