If you're learning Czech or would like to start, I can recommend following dictionary: http://cs.bab.la/slovnik/anglicky-cesky/
Besides Czech-English babl.a dictionary offers translations in 22 languages. All entries are provided with grammar, pronunciation, synonyms and example sentences. The dictionary is interactive and is based on the principle of Wikipedia - every user can contribute and suggest a new entry. The bab.la team guarantees the quality by verifying every new suggestion.
There are also quizzes, games and a language forum.
I just thought this link might be useful.
Good luck and have fun with learning languages!