The Diablo 2 Wiki
The Diablo 2 Wiki archive is available for anyone looking for information on Diablo 2. All information here is pre-D2R but contains a lot of useful information that is still relevant. Updated sections for new D2R features can be found on the PureDiablo Diablo 2 section


From Diablo 2 Wiki

The piercing property is identical in function to the Amazon skill Pierce; it gives a projectile a chance to pass through one target and continue on to hit a second (or third, or fourth, or fift) for full damage. Bonuses to Piercing stack with the Amazon's Pierce skill.

Numerous unique bows/xbows have 100% Piercing as a property, and you can find +33% piercing on the Unique Razortail Belt as well as several other items.