Æ

On Macs it's Option-' for the lower case æ and Option-" for upper case Æ.

On PCs it's ALT-0198 for Æ and ALT-0230 for lower case æ. The numbers have to be entered on the keypad on the right and not on the typewriter row across the top.

You can also do Æ and æ for Æ and æ respectively. These work on either type machine.