IM Booster is a free add-on for Windows Live Messenger and Yahoo! Messenger. Made by IMinent, IM Booster has been updated to include what they call Buddy Bubbles -- basically text in your IM messages that appears in a bubble, themed with a clever character. While it's cute, it's currently limited to only Windows Live Messenger (WLM) support and their own pre-defined bubbles. You can type your own messages in those bubbles, and they hint that in the future you'll be able to create your own bubble designs.
Here's a brief video demo of the new Buddy Bubbles in action:
Fortunately, IM Booster also provides other functionality, including libraries of custom emoticons, the ability to create your own photo emoticons and interactive avatars.
You can read more about IM Booster, the new Buddy Bubbles and the other features in the
IMinent newsletter.
7:30am PDT CORRECTION: As noted in the comment below, you CAN type your own messages in the bubbles. It's the bubble design itself that is currently limited to the predefined designs.