A new social bookmarking engine called TagTooga was released just today, so it's still lacking content. However, if you're a programmer interested in AJAX, Web 2.0, Javascript, etc., it might be worth your time to take a look. The site was designed to function as much like an application as possible. Once you get past the login/registration, there are virtually no complete page reloads. Clicks to edit, search, etc. cause inline divs to pop-in. The XMLHTTP calls to the server are such that the responses are instructions on what to do: update a div with HTML, show an alert message box, eval Javascript code, etc. So a single XMLHTTP response might update a couple divs in the DOM, run some Javascript to update the client-side state, and show an alert.