Really quick question, identify this component?

[Ride The Snake!]

Hello all, I'm just wondering if someone can quickly identify this component found in the picture on the following link:

http://carphotos.cardomain.com/ride_...0009_large.jpg

What is the round component sitting on top of the rear end of the passenger side valve cover? This is not my motor, but my truck has one too in the same location. Can't figure out what this thing is, but it can be turned by hand and doesn't appear to be threaded into the cover. No vacuum lines or anything attached to it.