It used to be...

Heirloom Chest, was 40 badges of heroism.
I think this translates in 2,175 current points, someone in trade was saying recently.

The Shoulders were the same price originally, so I will guess also 2,175 points.
But you could also get these from Wintergrasp with marks which were awarded for winning Wintergrasp.
There might be Tol Barad equivalents, not sure.

These function from 1st through 80th level, as far as the experience boost goes.
And the stats scale with level, getting better from 1st to 80th.
Also if you buy Plate Chest, then classes who are plate wearers from 40th and beyond can wear them from 1st to 39th as if they were Mail Armor.



Each of these was a 10% reduction (they say 10% bonus) to experience.
The math is a reduction.
Normally need 100% experience, with one you need 90% of normal experience (1.00 - (100 x 0.10) = 0.90)
But with two you need 81% (0.90 x 0.90 = 0.81).



There are other heirlooms.
You can get a Ring from the Dalaran Fishing thing, I believe 5%.
You can get a Cloak for 10%, from Guild Rank 12 and decent faction with your guild.

There are also 5% and 10% experience boosts, from guild level.

These all stack with the heirlooms, in the same way.
IE, multiplication, not additively.
And the Cata ones benefit you from 1st to 85th, not 1st to 80th.

Wasn't aware of trinkets.

I know there are other BoA heirlooms like 1H weapons or whatever.
No +experience on these, but they are strong items for your level.
Maybe the equivalent of having Blue(rare) gear that is of your level; not the best for your level ever really, but pretty close.