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.

I think if people are already here, it's more than safe to mention matching donation programs. It could actually really help motivate people. I know it helped me a lot in the past.

I once donated $3k (the limit of my previous employer's matching program) to local service charities in Austin, TX. The only reason I started investigating charitable giving in the first place was because I found the info about the matching program buried in the packet of info I got from HR when I was hired (which I got around to looking through 6 months after starting). My goal at the time was barely altruistic. It was some mix of "Cool, I can get $3,000 in extra money! I just need to find something else besides myself that I care about." and "Wow, I work for a government defense contractor. I know what they will spend that $3,000 on if I don't find something better!".

I don't think Less Wrong or Give Well existed at the time. My search for a good cause probably ended prematurely, but it still marked the beginning of a search for something outside of myself that I cared about.

Also, even though searching through information about giving to charity and strongly considering giving did almost nothing for me, actually giving that $6,000 changed everything about how I saw myself.

Sorry, I have to disagree on matching donations.

By switching a matching donation from third-world aid to existential risk mitigation, you do double your impact.