Arch Chinese offers quite a lot of features (Worksheets, Flashcards, Pinyin Table, Radicals, Tools), but the most useful thing is still the dictionary. It offers sound, stroke animations and, most importantly, it allows you to see in which words a certain character appears and how common that word is (also sorted on HSK level). Excellent for looking up character collocations if you want to find more examples of where a certain character typically appears. There are also several useful tools for teachers here.

More information about dictionaries here: 21 essential dictionaries and corpora for learning Chinese.