I would like to know if there is any "mouse-over" dictionary like Rikaikun/Rikaichan that can be used on other softwares other than a browser, like Word or even a japanese mmorpg (Who am I trying to fool? The latter is the real source of my interest, lol!). Thanks in advance.

[s0apgun]

http://www.kanjitomo.net/

Interactive Text Hooker.

The two mentioned above are excellent, I use them all the time. A great solution for Android phones and tablets is Jade Reader.

There's also Yomichan, an Anki plugin for reading text files (obviously with great Anki integration for mining).

Stardict. Will probably work in Word but not in an MMORPG.

Thank you all for the wonderful suggestions! I only had opportunity to try Kanji Tomo so far, but it works pretty well!