Not many people know how much money you can get off of rare spawns. If you have a lot of alts that are mixed levels this is a good way to log in each day and make quite a bit of gold. I use to use this method a lot, but I have stopped using it (until now).. There are so much rare spawns in Azeroth that it's just insane due to everyone skipping to the outlands.
A list of the zones that I go to for rares:
Name of Zone / Ammount of Rare Spawns
Badlands - A total of Six rare spawns.
Searing Gouge - A total of Six rare spawns.
Silithus - A total of four rare spawns.
Dustwallow Marsh - Nine Rare Spawns
These are the only spots I am camping ATM but I am trying out some new zones every day. The reason why I camp these zones is because no one goes to these spots.
After about two days of camping these spots I have only gotten some greens, the rare pet that dropped off "Dart". A lot of times the spawns won't be there (hence rare spawns teehee) but if you wait a couple days and log on your characters a lot of them are bound to be there.
I have one of my alts (level 60ish druid) logged in the badlands and he goes across the badlands to searing gouge when ever I log on him to check rare spawns. My other character (level 46 huntard) is logged in dustwallow marsh. And I go to Silithus when I do the Venomhide dailies on my main ;-P.
Tips:
After doing this a while I have found some things that have helped me out a lot and I decided to share.
Cartographer - This addon is really nice. If you hold down Control+Shift+Right on a area you can add a note on that location. You can add one of these notes on the map where the rare spawns are so you can know where to go with out alttabbing or anything like that.
Cartographer - Addons - Curse
allakhazam - This well show you where the rare spawns are. What I do is I just googled "Rare Sapwn (Name of Zone here)", then just find the allakhazam link to it (should be the first few using google). If you scroll down on allakhazam's page it well give you a list of mobs that are in each zones. If you look to the far right of where the mobs are it well show you what type of creature it is. If it says 'rare spawn' then it's a rarespawn =P. I haven't seen all of the rare spawns that are listed on allakhazam yet so I don't know how reliable it is.
Enjoy.