If you don't know the words to a song, you can always hum. But what if you don't know the words to what you're reading on the web?
Then you need a dictionary!
The instant dictionary extension for Opera not only provides instant explanations of highlighted words in English – it also works as a translator.
This function is especially useful if you are learning another language. In the default settings, you can set your native language, as well as your "target" language (the one you are learning), so the extension will automatically translate in that direction.
You can read more about this extension's features on the developer's blog.
Helpful and fun to use, Instant Dictionary is the definition of a great addition to Opera.