From virus removal and disk cleanup to
replacing screens/keyboards and
dusting out those clogged vents

Upgrade your RAM to increase speed (laptops included)
Install the latest version of iLife or Entourage while keeping your old settings

Set up a Multimedia server for sharing files across your home network
Sync your iPhone with a different computer
iTunes support

Set up a video conference
Access your Windows (or Mac) based network
Connect with your Exchange server