Google just rolled out functionality to allow the installation of gadgets in your Gmail (just click the labs icon at the top of you Gmail by "settings" to install it). This means you can add your calendar for a more consolidated view of your life, or even add something like tasks -- a feature that people have been wanting since Google Calendar was launched. Until now, people have relied on greasemonkey scripts like "Remember The Milk" for a task list.
This feature is one that I'm sure users will enjoy -- it gives them a good deal of flexibility needed to add functionality to their Gmail account.
Philipp Lenssen is right when says that this functionality, along with the labs project that lets you move sidebar items around, work very nicely together. By default, these gadgets appear below chat -- but in my case, I think I prefer it higher than that, so I was able to drag it there with that other labs project.