xsay is a notification tool for X.
If there is already a message displayed, the entered message will be queued and displayed once the user acknowledges the current message (for example, by clicking on it, or by running "xsay -d").
While multiple messages queue up, the bubble becomes bigger (as it is probably more important they are read). It shrinks again when messages are dequeued.
xsay also comes with a Gaim plugin, "gaimsay" that notifies the user when there are new instant messages waiting. This plugin also automatically dequeues the appropriate messages once the user focuses the appropriate Gaim window or tab.