Although it’s not a website, for English-Esperanto I highly recommend John Wells’ dictionary. It is available on paper from Amazon or even as an eBook on SmashWords. If you get the ebook version you can keep it on your phone or your laptop and it’s almost as convenient as WordReference for looking up words.
Otherwise there is the Reta Vortaro which is a dictionary that has translations into many languages. However the translations are more useful just to understand the meaning of the Esperanto word and it doesn’t really have enough words to be useful to look up a word in the other direction.
There is also a good multi-lingual dictionary on Lernu.