Most common oil leaks?

What are the most common leaks from the lower half of the engine? Rear main seal, oil pan seal, gearbox input shaft, drive shafts or clutch slave cylinder? I'm trying to locate a leak which seems to come from the rear of the engine, but I'm having trouble. I'm thinking RMS since oil is dripping out from the split between the bellhousing and the rear of the oil pan.
 
i would probably say rear crank seal, if the oil is relatively clean it would more likely be gearbox oil, check your gear box oil level.
 
Thanks. I'm pretty sure it is motor oil. It does not have the hydraulic smell of gear oil. Must be RMS then. I guess I'll have to live with it until it worsens. I'm not in the mood to pull the box now.