Here are some Munchkin cards that I've been thinking about for a while. I am always most interested in cards which simultaneously have benefits and drawbacks. While cards which are purely deleterious are kind of fun if you can compel others to use them, cards which are deleterious but used anyway because of the twisted psychology of a munchkin are far more entertaining.
Break Down a Door: Draw Three Cards
Discard this card to draw three door cards face down and put them in your hand.
This card addresses the fact that there's no way to straight up replenish a depleted hand, despite that fact that many abilities are powered by discarding cards.
Curse: Draw Three Cards
Draw three door cards face up. All curses drawn this way effect you and all monsters must be fought as if you broke down a door. If your are already engaged in combat, any monsters drawn join in the fray.
The substantially more dangerous and wacky version of the preceding card. This curse could easily be seen as beneficial as it essentially allows the target to engage in an additional combat; however, there is serious risk in self-application as any curses drawn take effect, and more/more-powerful monsters than are easily handled could be drawn. Such a curse may also make a munchkin rue the day they equipped a curse-reflective helmet.
Loot the Room: Pendant of Pedantry
When a player engages in pedantry roll a die. On a one, the pendant explodes and kills the munchkin that owns the pendant along with any munchkins that may be helping her in combat. On any other roll, give the pendant to the pedantic munchkin and gain a level. This item cannot be sold.
Whether this item encourages pedantry or discourages it really depends on the perceived risk/reward balance and the number of (re)loaded dice available to the players. This may simply introduce chaos into the game, but that's what Munchkin is all about anyway :P.