The new Microsoft reader adds a feature that makes their dictionary useable -- a word lookup on the front page. Before they did this, it took forever to look a word up in their dictionary. In their free ebook section, they also have dictionaries for several lanugages. I use the French-English and the English-French (two different books). When reading an ebook, you can switch dictionaries. I would never read a French ebook, but it's nice to know the capability is there.
For dictionaries that aren't free, I still like
Lextionary