PCs:

Microsoft Internet Explorer 5/6/7 - press 'Alt' and letter (or number), then press enter
Netscape 6 - press 'Alt' and letter (or number) at the same time
Mozilla Firefox - press 'Alt' + 'Shift' + letter (or number) at the same time

Macs:

Microsoft Internet Explorer 5 - hold down 'Control' key and then press letter
Mozilla Firefox - press 'Control' and letter at the same time
Safari - hold down 'Control' key and then press letter

Keyboard shortcuts
Keys:

S - Skip to the main content of a page
0 - Load accessibility page
1 - Load home page
2 - Load the contact us page
3 - Load the site map