This site combines the best of a thesaurus and a concordancer. A word search produces loads of combinations in actual usage, with a red or green bar to indicate good and bad connotations. An alternatives link shows other words that can be substituted. Should be a nice accessory to non-native more advanced writers. Also connects to Wordle, so that students can see visually what kinds of usage are most likely.
This remarkable concordancer has many quick and easy features, such as seeing visually with a graph the frequency of occurrences of a word, quick links to the word embedded in a concordance, a thesaurus of alternative vocabulary, and indications of "good" and "bad" uses of a word. Stannard doesn't talk much about the pedagogy of the tool, but it is well worth exploring, esp. with your more advanced students.