I have to chime in with a pitch for Castrol Super Clean. Comes in a purple bottle. Much cheaper to buy the gallon refill jug if you can find that. ($6.99 a spray bottle, but only $9.99 a gallon.)
It'lll take brake dust off car wheels... anything that can do that is ok by me. Fantastic degreaser. Be sure to rinse it well if it's on painted parts or aluminum.
I also use simple green. I get cheap plastic containers with lids from the dollar store and soak them overnight. Any leftover residue comes right off with an old toothbrush.
I used simple green and separately, kerosene. Both worked great. The kerosene was faster and better on the heavy gunk. The Green was really good, and better for me and the environ for average dirty parts.
I had a rear hub completely frozen with hardened grease and dirt from the 1930s and it was litterally rock hard. I had tried WD-40, PB Blaster, and a citrus degreaser and none of those worked. I read on a hot rod forum to soak the part in Pine Sol... I soaked the hub in a small bucket for a couple days and it seamed to do the trick... Im guessing simple green would have the same effect though as a degreaser... I just happened to have an old bottle of pine sol under the sink anyway...