[MunchkinMan]

You can play the following card types from your hand based on the following rules:

Classes/Races/Loyalties/Accents/Mojos/Factions/Birthrights/Armies: At any time during your turn (even during combat) or as soon as you get them.
Styles/Powers/Training: At any time during your turn (even during combat) or as soon as you get them.

GUALs: At any time, on any player, even during combat.

Items: At any time during your turn or as soon as you get them (except during combat) unless they are one-shot Items (usually says "usable once only") which can be played and immediately used for either a player or a Monster in any combat, or whenever it is appropriate (e.g., the Loaded Die can be played by the person who just rolled a die). Some Treasures do not have a Gold Piece value printed on the card, and are therefore not Items, but work very similarly.
Steeds/Vehicles/Ships: Steeds, Vehicles, and Ships are Items, and so therefore follow the above rules, however you may normally only have one Steed or Vehicle, and one Ship, at a time.

Hirelings/Sidekicks/Mooks/Minions/Allies/Comrades: At any time, on yourself only.

Monsters: With a Wandering Monster card (or the like) during any combat or in any Room (Room cards are found only in Star Munchkin 2), or during a combat where special rules are in effect(e.g., Bat Rules in Munchkin Bites!, or Hong Kong rules in Munchkin Fu), or when you are Looking For Trouble (i.e., if you choose to fight a Monster from your hand if you did not find a Monster when you opened a door).
Monster Enhancers: Any time during any combat (with some notable exceptions, like Enraged and Friendly can't be played together). Or, at any time on a Mook, Monk or Epic Werewolf, according to the rules for those cards.
Enhancer Enhancers: At any time during any combat where a Monster Enhancer that adds or subtracts Levels (i.e., you can't play Very on Mate) to a Monster in combat.

Rooms (found only in Star Munchkin 2): Any time someone kicks down a door and does not get a Room card, a Room may be played immediately from anyone's hand. Once a non-Room card is played on a combat, a Room may not be played.

Curses/Traps/Disasters: At any time on any player. In other words, on that guy next to you right now, or on yourself next Tuesday, while you're in combat with a pack of crazed Maul Rats, if that's what you want to do. However, note that playing a Curse-type card while another player is in the middle of completing an action does not interrupt that action; he may complete it first. (Just saying he's taking an action is not enough; he must be discarding or playing cards, or rolling a die, or otherwise making an affirmative step toward doing what he wants to do.)

Portals (found in Munchkin 6, Munchkin Cthulhu 4, Munchkin Zombies 3, and Munchkin Adventure Time 2): At any time on your turn except during combat, but you may only choose to play one Portal per turn. Portals you find when Kicking Open The Door all take effect. Whenever you play a Portal, draw a Door card to replace it -- face up if the Portal was face up (usually only when Kicking Open the Door), face down if the Portal was face down (usually only from your hand). Note: The rules for Adventure Time 2 are slightly different, and you should follow them when playing with that set and no other Dungeons sets.

Special notes:

For cards not covered here (and even the ones that are), "[A]ny time" without any other qualifiers means any time, even during combat.

One-shot Items may be played to the table and carried (they are items, after all). If they are carried, there is no change on how they may be used in combat. Under normal conditions, anyone can use a combat oriented one-shot Item during any combat.