RareScanner is an addon for World of Warcraft that includes a group of tools to help you track rare NPCs, treasures and events while you play.
The notable tools that RareScanner includes are:
Alerts from nearby rare NPCs/treasures/events
This is the main feature of RareScanner. The addon displays a button in the middle of the screen with information about all the entities detected nearby. Around the button information related to the entity is displayed, such as a miniature (in the case of an NPC), the name of the entity and the loot that you could obtain after killing/collecting it.
World map information
RareScanner adds icons of every rare NPC, treasure and event that it tracks to your worldmap. When you hover your mouse over these icons you will get extra information in a tooltip.
You can customize all the information displayed in the map and the tooltips from RareScanner options panel.
Rare NPCs explorer
RareScanner is designed to help you track every rare NPC/treasure/event in the game, but usually you will be more interested in finding NPCs that drop useful loot, like missing mounts or pets that you want to add to your collection.
While you can use the world map to check the loot of every NPC/treasure by hovering the mouse over their icons, it will take a lot of work to find exactly those missing items, so here is where the explorer comes in handy.
RareScanner allows you to add your own NPCs (even if they aren't rare), so you can get alerts whenever you pass by them.
Out of the box RareScanner will be tracking every entity in the game, however you might be interested only in a few. Using the previous tools (the world map interface, the explorer, the button with the alert or RareScanner options), you can configure the addon to track only for what you are interested in.