Sorry, I can't help by offering a link for a site like that, but I do think you would be better off just getting a cheap pocket dictionary and looking stuff up on the fly. Translating sentences by computer rarely if ever works well between Japanese and English due to a lot of things, but particularly due to the difference in degrees of abstractness--English is a whole lot more concrete than Japanese.
Anyway, such a site would be fantastic for simple words, but you shouldn't realistically expect to be able to translate whole sentences well. It might work once in a while, but, for the most part, it'll just come out garbled.
Plus, if you get a decent dictionary, it'll provide example sentences so you'll know exactly what context you can use that word in. A lot of meanings can be tricky if you don't have any examples to work with.
Good luck! ^.^
Edited, Mon Jan 26 10:44:12 2004 by Elulu