If engine oil, off the top of my head from top to bottom (and probably in order of likelyhood):

1) Valve cover seals
2) Head gaskets
3) Front or rear main crankshaft seals
4) Oil Pump gasket
5) Oil filter or oil filter adaptor gasket
6) Oil pressure sending unit
7) Engine oil cooler lines (move to #1 if equipped)
8) Oil pan seal
9) Oil pan drain plug and/or plug gasket
10) It is also not unheard of to blow oil out of the filler neck, distributor adaptor, dipstick tube, PCV valve, or blowby hose if it's really nasty inside. <img src="/forums/images/graemlins/barf.gif" alt="" />