SilverDragon

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.

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.

A LDB plugin is included to list rares in your current zone. You'll need to install an LDB viewer to see this -- I like ButtonBin. (You could use Broker2FuBar to put it on FuBar.)

If you have Cartographer addon or HandyNotes installed it will add map notes for rares, which can be shared with others.

Note that if you don't have a Broker viewer, and you don't have a supported map notes addon, all this addon will do is print a message into your scrolling text when you mouse over a rare!

SilverDragon 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. )

2.2.2
Adjust said targeting frame a bit.
Fix a bunch of errors I found when looking at the Scarshield Quartermaster.
More feedback when creating a macro.

2.2.1
Show in the tooltip whether a mob is already in the unit cache.
Option to choose what ways of seeing a rare will make the targeting frame appear.
Bugfix for the click-to-target options not showing. (Thanks to EthanCentaurai!)

2.2.0
Add click-to-target frame that pops up when a rare is seen.
Add macro-generator.
Configure which sound is played when a rare is found. (Thanks to EthanCentaurai!)
Options for which scanning methods to use.

2.1.2
Quick fix for a nil error