It's DRM-free in the Humble Store as well. Look for the DRM-free icon on the right. I'm pretty sure it's also without DRM on Steam.
Friendly DRM systems like the AppStore or Steam might seem a good idea until one gets burned by them.
I should know, still haven't learned my lesson.
The old UFO/XCOM games for example are sold through steam, but ship as a DOSBOX.