One of my favorite things to do is read. These days, I mainly read online, articles I aggregate from various technical and news sources. I also love to write and to try to harness the exact words I need to convey what I am thinking. I find that I always want/need new words to express my ideas. And, one of the ways I grow the word base is to look up any word that I happen to come across that I do not understand.
Traditionally, I have just used my dictionary.com firefox search add-on to look up words. I would command+t to open a new tab, search for my word, and close the tab to resume reading. But, today I just installed Dictionary Tooltip, another add-on for Firefox. It allows me to highlight words on any website and pull up a definition for that word in a little pop-up. This is very nice, as you don't need to leave the page you are reading.
It seems that there are several ways to pull up the definition window - command(right)-clicking, a keyboard shortcut, and clicking a little dictionary icon that appears whenever you highlight something.
I'm not too familiar with the dictionary site that it uses, but it seems effective, nonetheless.