A question sprung into my mind today, whilst watching soccer am, can anyone tell me what purpose the semi circle on the edge of the 18 yard box serves? Whats it for?

From wikipedia:-

The penalty mark (or "penalty spot") is 11 m (12 yds) in front of the very center of the goal; this is the point from where penalty kicks are taken. The penalty arc (colloquially "the D") is marked from the outside edge of the penalty area, 9.15 m (10 yds) from the penalty mark; this, along with the penalty area, marks an exclusion zone for all players other than the attacking kicker and defending goalkeeper during a penalty kick

Ah.....so...its specifically used as a marker for Penalties then. Cheers Gringo

Basically yes, as you have to be 10 yards away from a free kick and at a penalty, you also have to be outside the box.