Firefox with Greasemonkey: We've shown you one way to display your Google Calendar in Firefox's sidebar, but it took a lot of work. Here's a user script that makes it a three step process.
We already know how to load any site in Firefox's sidebar, but when you try that with Google Calendar, you can't actually see the important part of the window. With the Mini Google Calendar user script, however, you can view your day's events as you browse. First, install the user script and bookmark Google Calendar. Then, right-click on the Google Calendar bookmark, hit Properties, and check the "Load this Bookmark in the Sidebar" box, and you're done! When you click on the Calendar bookmark, it will load in Firefox's sidebar, showing you the day's coming events.
Note that if you want to open up Google Calendar in a full window, you can hit "u" on your keyboard to toggle back to the full Calendar view instead of the "mini mode" required by this trick.