The latest release of Trillian (which is an Instant Messaging client that allows you to run AIM, Yahoo!Messenger, ICQ, IRC and MSN Messenger from one app) has a new feature that underlines certain words as you chat.

When you roll over the underlined words, it gives you the wikipedia definition and a list of related URLS

If you click on the word, you can copy the definition and search the web (it automatically uses My Way which gives you a tabbed search page with Google (default), Yahoo!, Ask Jeeves and LookSmart results.