**De Vaan**'s *[Etymological Dictionary of Latin][1]* is perhaps the most up to date one. He also bases his articles on the other main etymological dictionaries, like Walde-Hofmann and Ernout-Meillet. Sometimes I would have liked his articles to be a bit more expansive, but that would have required a lot more time to write. I think it's normally only available on paper.

  [1]: http://www.brill.com/etymological-dictionary-latin