Zombie

There are numerous forms of zombie battles, which share certain similarities. Zombies cannot run, and may even be restricted to shuffling on their knees. They also usually must chant braaaiiiinnns constantly to make it clear that they are zombies. Depending on the particular battle, new zombies may be created by specific fighters (necromancers), by being killed by a zombie, or by dying in any manner. Most zombie battles end with all fighters being zombies.

Necromancer Battle

Fighters: Any reasonable number. A good ratio would be one necromancer to every 6-15 zombies.
Field: Not too large. All fighters should start relatively close together.
Regen: A necromancer may touch a single dead fighter at a time and call out "3, 2, 1, [necromancer's name]", at which point the fighter arises as a Zombie and chants the name of their Necromancer. Zombies may not be killed until they are standing or raised until they are lying on the ground.
Teams: As many as there are necromancers, see above.
Rules: All fighters, save the necromancers, start out lying on the ground and spread out in a roughly even fashion (corpse osmosis). The Necromancers start at opposite ends of the field. When the herald calls "Lay on", the necromancers can run into the field and begin raising their zombie hordes. Fallen zombies can be raised again, and a Necromancer can raise another Necromancer. When all the zombies belong to the same Necromancer they turn on him and, after killing him, the battle is over and new Necromancers may be chosen.
Tactics: Zombies have no real tactics, they're zombies. Necromancers have found it advantageous to ignore the corpses nearest to them at first and start by raising zombies in the middle of the field. This allows you to establish a front line, to be able to more safely raise the corpses in the backfield. Necromancers will also find their unique ability to run to be highly useful.
Variants: Instead of choosing necromancers, just choose a number of necromancers, and begin with a free for all. As people die, they fall and lie down in place. Once there are only [the chosen number] of fighters left, they become the necromancers. Usually in this variant, when fighters become necromancers, they are healed and must move to opposite sides of the field.
For added chaos, don't let the necromancers call out their name when they raise zombies. In this variant, each zombie must either figure out who raised them, and which other zombies are on their team, or just fight each other and the necromancers randomly. Often with this variant, it is good to give necromancers points of stoneskin so that they don't die too fast. This game ends when only one necromancer is left alive.

Village vs. Necromancer

Regen: The one necromancer may touch a corpse for 3 seconds to raise them as a Zombie. Zombies may be raised multiple times. Villagers are not replenished (though they can return as Zombies).
Teams: At the beginning, perhaps 2/3rds or 3/4rs of the fighters are Villagers and the remainder are Zombies. The Zombies collectively and secrectly choose one of the Villagers to be the Necromancer. All of the Villagers must then close their eyes and spread out, and the Necromancer is secretly informed of their status.
Rules: The Villagers want to kill the Zombies and the Necromancer, the Zombies want to kill the Villagers and protect the Necromancer. As long as the Necromancer is alive, they can continue to raise new Zombies, so the Villagers must detect and kill the Necromancer as well as all of the Zombies, while the Zombies just have to kill all of the Villagers. Zombies, of course, can't run. Villagers and the Necromancer can (and should).

Zombie River Battle

Field: A field with a wide section marked out across the whole middle. There does not need to be much space on the "banks" of the "river".
Regen: Anyone who dies is raised as a zombie for the next crossing. Lost limbs and damaged bows, armor, and shields are not regenerated until the poor soul is killed and returns as a zombie. Zombies fully regenerate between crossings.
Teams: None, exactly. Zombies versus the living. One or more people starts out as a zombie, a sufficient number that they can at least somewhat cover the width of the "river" without everyone easily escaping.
Rules: All the living start out on one bank of the river. When the herald calls "Lay on" or "Cross", they all try to get to the other side. The Zombies want to kill the people crossing. Any people killed by the zombies become zombies themselves, wounded and killed zombies are restored to full health, and the herald calls for another crossing. The last person not a zombie wins. Zombies can't run.

Zombie Cutthroat

Regen: All killed fighters can go to a designated spot and resurrect as zombies. Zombies don't regen.
Rules: Starts as a regular Cutthroat, but dead fighters can regen as Zombies, shuffling along and calling for sweet, sweet brains. Since Zombies don't regen, it is possible for the living to win, but it's always still a cutthroat. Usually once the living start to be horribly out numbered they may form alliances, but this merely delays their inevitable doom.

Zombie King

Field: As large as you want it.
Regen: Anyone who dies is raised as a zombie after a count to 30. When the Zombie King dies he/she is raised after a count to 20. Damaged bows, armour, and shields are not regenerated until the person/zombie is killed. Zombies regenerate limbs at a count of 10 per limb.
Teams: Zombies versus the living. One person starts out as the zombie king.
Rules: All the living start out on one side of the field and the zombie king on the other. When the herald calls "Lay on" the people rush the zombie king. The Zombie(s) want to kill the people. Any person killed by the zombies become zombies themselves. The last person not a zombie wins. Zombies can't run or use missle weapons. Zombies who run or use missle weapons die and any hits scored on the living do not count.
Extra Rules: One Person can become a Cleric who can "cure" a dead zombie and resurrect that person as a living person. Must touch the dead zombie for 20 seconds to "cure" and resurrect that person to the living side. Can not "cure" the zombie king.

Zombies

Regen: Anyone who dies walks to the respawn tree and then enters play again as a zombie and zombies can continue to walk to the respawn point in order to respawn for most of the game (see rules). Other than that there is no regen until "Last Man Stand".

Teams: Zombies vs. the living (The zombies should be about one-tenth the number of people playing).

Rules: The living start twenty to thirty feet from the respawn point (preferably a tree) and the zombies start at the respawn point. Zombies can't use missile weapons and have to make "zombie sounds" (i.e. "Brains") as they move. Anyone killed becomes a zombie. Zombies can not die from the "two limb" rule, but they do not regenerate either. Once there is only one person remaining the call "Last man Stand" goes out. When this happens, the "last man" regens all his limbs and armor, and the zombies can only use single blue. Also, at this point zombie deaths become permanent.