jQuery UI is working on templating and also has a list of existing template engines (on that page).
If you want something super simple, just write something small to ajax in the content you want. Then include that script on all pages that need it.
$.get('/header.html', function(data) { $('#headerPlaceholder').html(data); });
$.get('/footer.html', function(data) { $('#footerPlaceholder').html(data); });