No more faffing about with external downloads and installer programs; the DayZ zombie survival mod for Arma 2 is now available through Steam. You can nab it from the relevant Steam page right now. As before, you’ll need a copy of Arma 2 and Arma 2: Operation Arrowhead to run it.
The stand-alone version of DayZ is still being worked on (this Steam release is just the older mod,) but you can read the latest update on that project at the game’s development blog.
None of this stuff should be confused with The War Z, which is an entirely different game that was pulled from Steam on account of its misleading feature list.