The basic layout of my site contains a horizontal bar of tabs at the top of the page that link to the various main sections. Below the tab is the content of each page. The tab bar in included in each page via ssi, so every page in the site references the same html document for the tab bar. Each tab is a separate image.

Question: is there any sort of code or script I can use so that when a visitor clicks on a tab and goes to the linked page, it will remember which tab was last clicked and display a different (e.g., differently colored) tab in its place (like for example, on the Apple site) so as to indicate which section the visitor is currently in, but while preserving the ssi included tab bar as only one html document?

You can do it in CSS.
Along with PHP, it is a cool combination.

Check out these references
http://www.codethat.com/phptree/index.html
http://css.maxdesign.com.au/
http://www.downloadfreetrial.com/scr...avigation.html
http://www.w3.org/Style/Examples/007/target.html

I would go with PHP. Writing a VERY simple script will allow you to have the script get the filename for the current page and determine what CSS classname to give to the tab based on it. As vivekar said, you can then use CSS to make one tab class (the "active" class, for instance) a different style (ie--color, bgcolor) than the rest.