They are a step up in difficulty in both content and ease of use than the previous group and are probably best suited to secondary schools. There is no reason though, why they cannot be used with experienced and capable primary school students. Easy to use. Relevant Fast Keyword or category search. Pure search engine.
Measures link importance.
Searches for exact match. I'm feeling lucky button. Directory of sites evaluated by an editor. Category search is fast and relevant. Part of Open Directory Project .