The Machine Mutants used this technique to move through the walls and ground of planet M-2.
The Sigma Force and General Rilldo use it in their battles against Goku, Pan, and Trunks. Natt uses this ability greatly when messing around with Pan, liquefying through another part of a wall when she was about to attack. The other Sigma Force members use the ability during their battle against Goku.
Super Buu liquifies his body for the Dangerous Liquid Bomb attack
Super Buu deliquifies to appear inside his own body