When you created a network of multiple sites in WordPress 3.0, you (the main blog admin) because a Super Admin with all kinds of fun powers. In some situations, you may desire to have additional Super Admins who can do things like install plugins & themes, perform upgrades, and manage global users and sites.
If you were using WPMU, not only did Site Admin change to Super Admin, the location of adding another one changed as well, so heads up.
Under the Super Admin -> Users menu, find the user you wish to promote. Hover over their username and a small “edit” link will appear beneath it. Click it. This will lead you to the user’s profile page.
I’ve obfuscated the username in this case, but here you can see the checkbox selected to make this user a Super Admin on my site. Hit the update button to save the chage, and the user now has access to the Super Admin menu.