http://lcorp.ulif.org.ua/dictua/ - has a very useful feature which you couldn't find in regular Ukrainian dictionaries usually. It's an example how you must change a word by cases (it's called Відмінки in Ukrainian) for each word. Another useful things are phraseology and synonyms, that help you to determine correct sense and use of a word.
http://slovnyk.net - another very convenient Ukrainian dictionary with cases, phraseology and synonyms. Unfortunately it doesn't work temporary. It's unknown when it will work: after week, month or later. Keep in mind it just for case.