GmailAgenda integrates your Google calendar in to your Gmail account. It works with both normal Gmail accounts and Google Apps Gmail accounts,  allowing you to see your calendar's events without having to leave Gmail.

The pane can be collapsed/expanded. You can choose which calendars are shown for each account. The pane can be refreshed manually at any time. There is a Quick Add link (not available for Google Apps accounts) that will allow you to quickly add events to your calendar.

This extension originated as a Greasemonkey script named GmailAgenda. It was made available on UserScripts.org and was eventually discovered.  This blog post was then referenced by Lifehacker and in one day the install count went up by 4,000.  With that many installs, the script, which was a quick hack, revealed all of its problems.  Work went in to fixing the problems but it was soon determined that the idea was bigger than the script could hold.  So with that, the GmailAgenda extension was born.