Google Calendar in Gmail

Think of all utility tools inside one box. Good right.
Now google enables to have calendar inside your gmail browser window itself.
Ealier to have calendar use, need to click "Calendar" link at top left corner of gmail browser window & it will open in new brower window. In this new window manage your calendar entries.

Now this legacy way is not required.

You can have calendar inside email window itself as below.

To avail this feature, do the followings

1. Click "Settings" link present at top right corner from your gmail window.
   
2. Visit the "Labs" tab.
   
3. Scroll to the bottom, select "Enable" next to the "Google Calendar gadget"
4. Click "Save Changes."
   

Source : http://gmailblog.blogspot.com/2008/10/new-in-labs-calendar-and-docs-gadgets.html