The Jefferson County cold case:

On the 27th January 1993 in Ina, Jefferson county, Illinois, the severed head of an unidentified woman was found in a brush in Wayne Fitzgerrell state park. 

The map below (from Google maps) gives us a general idea of the what the area looks like.

 Two little girls discovered the head while playing in the park, according to investigators the perpetrator made no effort to conceal the head on disposal.The general consensus is that it was probably thrown from a moving vehicle and is considered a murder.

 As you can tell by the reconstruction, the face was definitely recognizable with what looks like brown mahogany hair in a grown out bob haircut with bangs / a fringe. A small mole on her left ear was noted.
Although I agree with most that the image does look waxy and ghoulish it was probably rendered directly from the postmortem image and therefore holds the most likeness.

The dental analysis revealed a possible change in economic circumstances (or just a possible disregard for dental care) as there was costly previous work performed, coupled with a more recent state of decay. 

The victim was Caucasian and her age is estimated anywhere between 30 – 50. (The only news clipping I could find said she looked around 30)

Jane Doe had also suffered from Torticollis (more commonly know as wry neck syndrome) which meant she may have had difficulty turning her head certain ways and might have often spasmed involuntarily. It's thought that her head was likely often turned to the left.
The only other information about the Jane Doe is that her neck condition could have been linked to a past accident, as she had what is described on the isp.state site as a "healed traumatic lesion on the skull". I'm assuming that's a skull fracture or some sort injury sustained in her life previously. 

(image source)

Her identity remains unknown. 

If you have any information regarding this case, contact Stacie Speith at the Illinois state police on 618-529-6500quoting case number LE: C93-0159 

Sources: wikipedia
FB cold cases