So, this has to be a mistake, but you might as well take advantage of it before it's changed: Dark Souls is currently 19 cents. Not dollars, cents on Steam. That's about as close to free as you're ever likely to get. This is as good a chance as any to play my favourite game of the last five years.
Seriously. Buy it now before someone changes their mind.
UPDATE: Looks like it's gone back up sadly. I bought a copy for two of my brother in laws, went to buy it for the third and BAM, back to $19.99.