The most common leaks I have seen are the front crank seal, the oil pan (both coat the exhaust Y-pipe), and the rear valve cover (coats the alternator).
I have heard of quite a few power steering pumps going out as well which would put red fluid on that side of the engine.
The easiest way to find out for sure which one is the biggest leaker is to degrease your engine in the dirty spot (out-a the gutter you freaks
), then check it once every few times you drive it.
