I meant to write about these a few weeks ago, but forgot. Jon Hicks has put together a few very useful bookmarklets. These are incredibly practical and can be modified to handle most any kind of word look-up (e.g., instead of dictionary.com or thesaurus.com, which he provides, you could just as easily create one for Google, Amazon, etc).
On top of the highlight-word(s)-then-press-button interface, he has also created one for links — if you are on a particular webpage and want to send the link to a friend, you simply hit the links bookmarklet (that you’ve dragged to your toolbar) and it opens up your default mail client with the subject set to the title of the webpage and the link included in the body. Good stuff.
I had actually planned to do the dictionary.com thing on my own, but he beat me to it. While he made these for Safari, I’ve found that they also work in Mozilla/Mac — I haven’t tested them beyond that.