Windows SharePoint Services V3 supports a new way for you to add functionality to your sites - "Features."

In V2, for instance, if you wanted to add a new menu item to the toolbar on a list edit page, you would probably have to edit the page in FrontPage, drop a content editor web part on the page to inject JavaScript, or modify the list's template.  Unless you modified the list template, you would then have to duplicate that for each web where you wanted the new menu item.  And what if you had a web where you didn't want the menu item?

In V3, you can develop "Features" which allow you to package up functionality (menu items, list event handlers, etc) and deploy them at the web or site collection level.  Even better, you can activate and deactivate features allowing you to easily enable/disable functionality.

Check out the WSS SDK for more info.  View the Feature section from the SDK online here.