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).
Screenshots/Possible Uses:
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).