Steam today quietly brought Left4Dead, the first of the "zombie apocolypse" series, to the Mac -- and put it on sale for $10, half off it's usual $20 price. The company also slashed prices on related bundles, such as Left4Dead combined with Left4Dead2 (which only just came out earlier this month for Mac users) for $15, and a four-license version of Left4Dead for $30.
