WowAce

StatPoints simply displays a stat summary in points on an item tooltip based on your personal relative stat values.
It even gives you a recommendation on what stat to reforge (and to which stat you should reforge it)

So you can easily compare to item by there stats and your relative stat values.

Here a little example for cats with the relative stat values from mmo-champion.com cat guide:

As relative stat values we use following:
Weapon DPS 4.18
Agility 3.31
Mastery 1.47
Crit rating 1.04
Hit rating 0.96
Exp rating 0.96
Haste rating 0.90

So an item like http://www.wowhead.com/item=62417 (Liar's Handwraps) would have a statpoint value of:
1078,29 Unreforged
1116,48 Reforged (-67 haste +68 mastery)

The stats on this item are:

233 Agi
149 Crit
169 Haste

So we now calculate 233 Agi * 3,31 + 149 Crit * 1,04 + 169 Haste * 0,9 (for the unreforged version)

Plans for the future:
- Recommend gems and calculate the statpoint value with and without gemming