GmailAgenda is a Greasemonkey script that shows your Google Calendar agenda as a sidebar in Gmail. The script uses Google Calendar's embedded view and it includes an option to add events to the main calendar. The sidebar can't be minimized, so it remains visible even when you don't need it.
An alternative way to keep an eye on the Google Calendar agenda is to add the iGoogle gadget in the sidebar, as explained in this post. The advantages are that you don't need the Greasemonkey extension, the sidebar is visible even when you don't visit Gmail and it can be closed.