The HTML accesskey attribute is very useful usability feature that is reminiscent of shortcut keys in standard desktop applications. This allows you to define particular key combinations for your navigation links (e.g. Shift+Alt+H for Home). Originally, these access keys used the Alt key, but they would clash with the browser menu (which also uses the Alt key, by convention), so they were changed to use Shift+Alt in most browsers.
Firefox, Safari, Chrome and Internet Explorer use Shift+Alt for access keys. The first three will follow the link when you press the key combination, while IE (always the black sheep of browsers) will just focus it, so you have to press an additional ENTER to follow the link. I don't know what the developers were thinking. What's the point of just focusing a link?
It is up to the site to make the access keys evident, often by underlining the particular character that triggers the link - a convention typical of application menus. However, by pressing the Shift+ESC key, the Opera browser brings up a full list of access keys in a particular page, even if they are obscure. Subsequently pressing the access key character (e.g. Shift+ESC+H) will trigger the link.
Juicy Studio has a really good article on the topic.