Ready for Powerleveling

Different heirloom vendors: http://www.wowhead.com/search?q=heirloom#npcs

Gold venors are:
Alliance [npc=6294] in Ironforge | http://youtu.be/DevqdYWGFJk
Horde [npc=6566] in Undercity | http://youtu.be/jWC4CB6QtjE

Awarded for collecting 15 heirlooms! After patch 6.1, all collected heirlooms are learned and stored as a "crafting recipe" in the heirloom collection tab. This tab can be viewed on all characters and you can click on any heirloom to craft it into your bags - it will even be labeled with <Made by X>!

Heirlooms can be obtained in many ways, but are most easily bought from [npc=6566] (H) and [npc=6294] (A) for gold. Both vendors sell varies heirlooms, including the tokens that can be used to upgrade your heirlooms to scale all the way to lvl 50.

The heirloom prices are as following and are NOT affected by reputation:
- Chests: 500g
- Shoulders: 500g
- Necklaces: 700g
- Trinkets: 700g
- One-Handed weapons: 650g
- Ranged & Two-Handed weapons: 750g
- Shields & Off-Hands: 500g
- [item=122338]: 500g
- [item=122339]: 750g
- [item=122340]: 1,000g
- [item=122341]: 1,500g
- [item=151614]: 2,000g
- [item=151615]: 3,000g
- [item=167731]: 5,000g
- [item=167732]: 7,500g

Finally, if you collect 35 heirlooms you'll receive the Heirloom Hoarder achievement along with the [item=122703], which is a mount usable from lvl 1!

Edit: Updated for patch 9.0.1 (SL)