SilverDragon tracks rares. It'll remember where you last saw a rare mob, and will pop up a notification message when you get near one.
It comes with a list of rares. Type "/silverdragon" to bring up the options, and use the "Import Data" button to bring 'em in. (It's not installed by default, in case people would prefer to keep the suspense alive. :D )
If you import the default dataset: the very first time you come within visual range of a rare, SilverDragon will tell you. It only works the first time because it's checking the game's mob cache, and once you've seen it the game will remember. Remember, this won't happen if you don't do an import.
You can clear the mob cache by quitting WoW and deleting creaturecache.wdb from the Cache\WDB\enUS (or your actual locale) sub-directory in your WoW directory.
If you're running Windows Vista or 7, your Cache is actually NOT in the WoW install directory. Instead it will be at C:\Users\(username)\AppData\Local\VirtualStore\Program Files\World of Warcraft\Cache
If you have nameplates enabled it'll scan through them to find rares nearby. Otherwise it requires you to mouseover/target a rare, or for your party member/pets to do likewise.
When a rare appears, SilverDragon will pop up a frame that you can click on to target it.
SilverDragon can also create a macro to target any rares that are known in the current zone. It's just that good.
A LDB plugin is included to list rares in your current zone. I like ButtonBin as a viewer, but there'll be a minimap icon if you don't have any viewers installed. (You could use Broker2FuBar to put it on FuBar.)
If you have Cartographer or HandyNotes installed it will add map notes for rares, which can be shared with others.