most likely the knock sensor, it's located on the passengers side of the block under a metal shield to keep the heat from the manifold off of it. It's right in the center of the block between two freeze plugs.
Hope this helps,
Greg
EDIT: Now that you have the pic posted it's definately the Knock sensor the one on the left in the picture. The one on the right is the Oil pressure sensor, and is located on the front driver's side of the block right above the oil pump when the timing cover is installed.