A small but growing number of third-party developers are producing gadgets for your Gmail inbox. First you'll need to enable "Add any gadget" in Gmail Labs. Then you can add any gadget by grabbing the URL from the developer and adding it under the Gadgets tab in Gmail settings.
Realistically, Gmail has limited real estate for gadgets: You can add a dozen, but the constant scrolling required for viewing them defeats the purpose. In addition to Google Calendar, I use the Evernote gadget, as well as an awesome service called Xobni (originally an Outlook add-on) that's currently in closed beta but worth getting on the waiting list for.