I think you and Louie may be talking about two different kinds of matching donations. The GiveWell post is about an employer matching donations only to a specific charity. Some employers will hold this sort of pledge drive, particularly in the wake of an especially harmful natural disaster.

However, many employers will match donations, up to a certain level, to any qualified (e.g., 501(c)(3)) charity; I believe one can find such employers by searching the database linked by Louie.