Wumpus Game - First noted by a GoogleRumors commentor, if you add the buddy wumpus.game@gmail.com you can play the classic text-based game. It took a little while for us to be approved but we’re proud to say that we just got eaten by the Wumpus.

Change the font size - While holding the control key, move the scrollwheel on your mouse either up or down. This trick works while being focused in either the read or write area.

Insert linebreaks - If you want to have a message that spans multiple paragraphs, just hold shift and hit enter. You can add as many new lines as you want to create.

Style text - Also noted by a GoogleRumors commenter, you can make text either bold or italic by surrounding it with * or _. (Example: _this_ is italic.)

Switch windows - Hitting tab will cycle through open windows. It will select minimized conversations, to expand them just hit enter. If you just want to cycle through IMs and don’t care about the buddy list, control-tab will do that and will automatically expand a minimized conversation if you settle on one.

(hat tip: chris)