Ever played a first/third person shooter? In most shooters, you will realise as you take damage your screen will redden, until you heal up again! The Bloody Screen Mod 1.4.2 adds this feature to Minecraft! The edges of the screen will also change colour when hit with any of the negative potion effects, even ones you cannot get a potion for currently (eat some rotten flesh!), with the exception of nausea and Blindness, because, you know, they mess your screen up enough already.
The Bloody Screen Mod 1.4.2 Installation: