BigBlueBall has a good feature on Google Talk. While technically interesting, these tips are most useful:
* You can change the font size by focusing your mouse cursor either on the message window or input box, holding the control key down, and spinning the scroll wheel.
* Use the keyboard: Pressing Tab cycles through conversations in each stack and the main window. Ctrl+Tab or Shift+Tab cycles backwards.
* When you see a message notification, you can right click it to close it without focusing the conversation window.
* To make something bold, you must place an asterisk before and after the area you want to bold: *example* turns to example
* If you want to italicize something, you must place underscores around the area you want to italicize: _example_ turns to example
* Between one set of ‘*’s or ‘_’s, you can have up to and including 100 characters. If you exceed this number of characters, the effect will not take place and you will get normal text with the * or _ symbols visible at either end
* Typing a smiley like 🙂 and 😀 in an IM window will make it turn bold blue like: 🙂 and :D. Other valid smileys are 😦 😛 :O 😐 😥 😡 🙂 😀 😦 😛 :-O 😐 :-x.
* Preceding text with a symbol from the hebrew alphabet, such as ? and ?, makes text bold and large
* Surrounding text with \\ and // makes text clickable like a link but does not lead to any url.
Download Google Talk here