I've found this Chrome extension, allegedly made by Google. You just have to double-click a word to view its definition in a nice pop-up bubble: https://chrome.google.com/extensions/img/mgijmajocgfcbeboacabfgobmjgjcoja/1264144028.42/screenshot/1
It works perfectly and is very well designed. I love it! https://chrome.google.com/extensions/detail/mgijmajocgfcbeboacabfgobmjgjcoja

PS: I guess Ionut already knows about it, or will be pleased to discover it – cf. "f you'd like to see an official API for Google Dictionary, star this issue." http://googlesystem.blogspot.com/2009/12/on-googles-unofficial-dictionary-api.html. 508 people starred this issue, by the way!

Oops, here's Ionut post http://googlesystem.blogspot.com/2009/12/on-googles-unofficial-dictionary-api.html (note that there's probably no official API for Dictionary yet, only an official Chrome extension for Dictionary).