I'm looking for an addon that I can add any ability or buff to and it makes a message on the screen saying I got like Clearcasting, or Brain Freeze, or whatever.

ScrollingCombatText will probably do the trick; I believe you can set it to show buffs and I believe it is very customizable at that, too. You can set it to like show certain buffs and such; it also has great cool-down alerts. Check it out, I think you'll like it.

EventAlert?

DoTimer has a feature called Notifications.  It is highly configurable and allows you to be notified in various, even multiple, ways of events.  This includes gaining or losing a buff or a debuff, such as Clearcasting or Brain Freeze.  I use this on my warlock to notify me when I gain the Decimation buff and have chosen two notifications from the many available... screen edges flash with green and it also plays a sound.  Completely unmissable, and I always know when it's time to cast a Soulfire.  DoTimer is a powerful set of addons that work together brilliantly, and I cannot recommend it highly enough.

http://wow.curse.com/downloads/wow-addons/details/do-timer.aspx