"Play soccer with the heads of your decapitated foes in Blood Ball!"--A Blood II: The Nightmare Levels Promotional
Head Soccer (or "Blood Ball") is a general term that can be used to describe a common trait in the Blood series of kicking the heads of the player's decapitated enemies. This originally started in the original Blood, with Caleb being able to decapitate the Axe Zombies and kick their heads comically across the room. The same effects were used to let players kick around pails as well. There was even a BloodBath map set in a soccer field and stadium with nets allowing players to play an actual game of soccer (although no score was kept by the game), as well as some of the mini-games inside the level E1M4: Dark Carnival which were dependant on head kicking.
A poster called Asphyxium has claimed that head kicking came about, in part, from his suggestions during the game's closed beta. The tradition of Head Soccer was continued in Blood II: The Chosen, where the heads of Innocents, Cultists, Fanatics, and even Soul Drudges (the enemies most similar to Zombies in Blood II) could be kicked around and bounced repeatedly up in the air. As part of the new features of The Nightmare Levels, a "Zombie Head Soccer" BloodBath mode was added to the game.
Although best known from Blood, this trait has also appeared in later games - most notably in Postal 2 where players can behead enemies and NPCs and play with them in a variety of interesting ways, with one of the most novel being the ability to use them to play fetch with friendly dogs. There is also a limited implementation of the trait in Quake 4, where the player can move around certain helmets that are sometimes left on the ground; whether or not any of these helmets contain decapitated heads is never established, though several can be found near corpses.
It should be noted that the Blood series did this before supplemental physics effects became widely popular in first-person games, such as in Half-Life 2 and Doom 3 with their gravity guns. Blood II also featured other physics features such as movable objects, which could even be dragged around by pressing the "G" key.
Transfusion and BloodCM, both fan recreations of Blood, also feature head kicking; as a homage The Flesh Game features Caleb kicking a zombie head after blowing it up with a remote detonator in the basement area. Part seventeen of the fan fiction piece Scroll is titled "Head Soccer".