One of the most glaring missing pieces from Gmail(Google Apps) is a
task list. In comes Remember the Milk(RTM). Here's the main website:
http://www.rememberthemilk.com but read this first:
http://creativebits.org/toolbox/to-do-list_for_google_apps_right_within_your_gmail and then go to the site.
There's a nice little addon for Firefox which is very easy to install after you sign up for a free account.
Also RTM works with about everything out there too, there's even a dashboard widget for Mac.
Words of Wisdom: If you can think of some missing functionality odds are good someone else has already thought about it too and done something.
Here's something fun from Google:
http://www.google.com/ig