Looks to be Arch running i3 (my own current setup as well, actually)

Looks like Linux with Openbox: http://openbox.org/wiki/Openbox:Screenshots

I doubt that's Openbox. It would be one of the many tiling / dynamic WM's out there (DWM, Awesome, i3, Xnomad, etc).

As for which WM specifically, it would be impossible to say for certainty since every one of them can be configured to look like the other - so only the OP would know for sure.

I'd speculate that you're right about Linux, but only because the article exampled was about ArchLinux. However that could be a red herring and the aforementioned WMs would also compile on most modern *nixes (or at the very least, a few of the notable BSDs)

archlinux (you could have guessed from the page shown;), wmii as WM, no customizations whatsoever.

Obviously false. They could not guess from the screenshot.

I did customize wmii default theme colour too. Twice falsified.