Regarding Force spirits however, the most classical example would be ghost-Obi Wan appearing in the amount of places he appeared during the original trilogy. Died on the Death Star, appeared on Hoth, then Degobah and finally on Endor with Yoda and Anakin - Force spirits just don't seem to be bound to the laws of the universe.
Originally posted by Ziggystardust
I'm not sure if all sith spirits could do it, but Freedon Nadd was following Kun around across System's, and his spirit almost certainly wasn't lingering around on Kun's starship.
That's Fold Space (and yes, all spirits can do it). Essence Transfer is a different power.
As I said, it was retconned:
Palpatine's body was destroyed. Separated from his clones, Palpatine was forced to survive in the maddening, bodiless existence of the void. Through sheer will he retained his identity, crossing the gulf of space to again take up residence in his clone body.
-The Complete Star Wars Encyclopedia