It said I was logged in as "Blogger!", but I have most certainly never seen your site before. So it seems your sessions/user/cookie/security system is broken.
Color scheme is violent.
The "about" page has completely different stylings.
When logged in, the "LOGIN" icon goes to "logout.php", you will likely need to dynamically rename it.
Whenever I select one of the "World/DE/FR/UK" settings it doesn't alter the search results, and the setting is forgotten when it loads the search page--forcing users to reset it continually is unpleasant.
You shouldn't be inlining your stylesheet, have a seperate CSS that you import.
There are several combined Scriptaculous/Prototype libraries that zip them together into one highly compressed file instead of two moderately large ones. Consider looking into that. (Further, you may want to consider doing this with only moo.fx, it is something like 3k instead of 150k, and load time for search results is not a trivial thing.)
The ajax displaying of the search results is very jerky, if you are expanding entries one by one, try expanding the entire division at once.. if you are already doing that, consider not using a visual effect on opening, just having it open responsively without the visual lag would be better (in my opinion).
Things I liked:
Access keys (have them more uniformly available)
The 10-20, 20-30 etc ranges you can click on to view results.
Ending question: why do I want to use your service instead of a Google / social bookmarking information cocktail?