ChatFlow is a simple mod that allows you to intercept any incoming messages using regular expressions.
To open the GUI, press F12 (by default, configurable in the keybinds menu, of course).
As of 1.1, you can now redirect chat messages to Toast Notifications!
I'm pretty sure you can come up with some good uses for this too :)
Figuring out how to use the mod can be daunting, so here's a tutorial:
If you have any problems with the mod or just want to contact me, you can find me on Twitter (@Vazkii) or IRC (#vazkii on irc.esper.net).