It's been 4.99 before, but this is the cheapest price for it yet. You get a DRM free version, and it also includes a Steam Key.
Frankly, it is worth $9.99 easily.
EDIT: Sale is over, but it'll surely get cheap again.
Edited by KJAZZ, 14 March 2013 - 05:12 PM.
Added end time for sale