One of the most time-consuming tasks when learning Chinese is looking up characters in a dictionary. This is especially true when trying to read a passage that has quite a lot of unfamiliar words in it.
There are some web browser extensions that greatly speed up this process, automatically displaying translations for words as you hover over them. Here are the ones I currently know about:
Perapera
The add-on is available for both Firefox and Chrome. It’s available in several languages (English, French or German to Chinese or Japanese, with Korean coming soon). The author also maintains a blog containing reviews of other language learning tools and material.
Zhongwen: A Chinese Popup Dictionary for Chrome
This add-on is similar but is only available for Chrome. One distinguishing feature is that it has integration with Skritter.
It looks like Safari and Internet Explorer users are missing out here. If you know of any more add-ons that do this kind of thing, let me know in the comments!
Update (2014-01-11): Frill
After recently switching my main web browser to Safari, I became frustrated with the lack of options and wrote Frill: a Chinese pop-up dictionary extension for Safari.