Integrated Gmail Firefox extension allows you to make the Inbox Collapsible and load your choice of Google Calendar, Reader, Notebook, Groups, Maps, Picasa, Sites, News and Portfolio plus Custom URLs (w/navigation buttons innframe) and Google Gadgets seamlessly into the Gmail interface below the inbox (rather than the sidebar) using a collapsible box. If your are inbox minimalist, enable minimalist mode to clean up the inbox even more.
Be aware that this does a lot of fancy Javascript work to make things happen, so a few Labs features will cause conflicts. I had enabled the ability to move nav columns around, which it didn't like. Also, even in settings you'll see Reader, Calendar, etc. So if you're not keen on seeing this in every single screen, you're kinda stuck with it.
