Lots of us have encountered those wide-eyed and eager WoW players who walk up to anyone with epic gear and say, "WoW! Where'd you get that? That's uber-cool lol!" We may even be that sort of player from time to time, if we see a piece of gear we think looks really wonderful or seems particularly useful.
WhoDrop answers that question for you before you even ask. Whenever you inspect another player (I'm using Examiner for that in the screenshot above), WhoDrop will insert a line at the bottom of the tooltip for each item you examine, telling you where exactly that item can be found. It'll tell you which boss drops it, whether it's crafted or not, or even how many Honor or Arena points it'll cost you to buy. Another addon which has a similar function is called Mendeleev, but I found that it overloaded me with too much information, so I went with WhoDrop instead. (Incidentally, the other two tooltip-info addons prominent in this screenshot are RatingBuster, with the stats, and Auctioneer, with the price information.)
Additionally, if you're approaching a certain boss fight, grinding a certain reputation, or PvPing for certain rewards, WhoDrop also features an expandable list of the items you can get from these various sources. It was also recently updated to include the Arena Season 3 gear, so you can now drool over that any time you wish. If you want a prettier interface just for looking at boss drops and such, you might consider AtlasLoot, which has most the same information and is easier to browse through (but it has no tooltip info -- you'll still need WhoDrop for that). AtlasLoot is a plugin for Atlas maps, which can help if you ever get lost in instances.
You can get WhoDrop at IncGamers, but I prefer to get mine direct from wowace.com, and keep it up-to-date with the WoW Ace Updater for the Mac.