Wiker

Wiker is browser extension which helps you to get basic understanding of unknown word. Just select a word, press Shift+1, and you'll get popup message with short reference of the word from Wikipedia. If the word is not in your language, the extension will translate the word for you using Yandex translation service.
If the word has several meanings, you can press extension button on toolbar and get all of it
The extension is rewritten What Is It extension for Opera 15+ and bugs fixes.