Rules for games I talked about:

99:

Players start with 3 cards and play on a community pile in the middle of the table.  Each card has a specific value and players add or subtract that value from a running total of cards.  The idea is to not be the player that pushes the pile over 99.  If you play a card that pushes the pile over 99 (which you are forced to do at times) you lose.  To build math skills, it is important that each student be required to complete his/her own math and state the pile's worth after they play a card.  Here are the values as I learned them growing up:

A: adds 1

2, 3, 4, 6, 7, 8,: Add their face value

5, 10: subtract 5 and 10, respectively

Jack and Queen: Add 10

9: Adds zero

King: Makes the pile 99

I like this game because it is what I call "portable," in that, you can switch the rules to fit your needs.  I actually taught it to one grade with different values (I think 10 was + zero, 5 was plus five and 3 and 7 were subtractions) because I thought the subtraction of non-five and ten numbers would be more beneficial.

Zilch:

Each player starts with 4 or 5 cards (four cards for the first few play-throughs, five later on).  Each card is worth its numerical equivalent (so A is 1, Jack is 11, Queen is 12, and King is 13) except black cards are positive and red cards are negative.  Players are trying to get their value of their hand to zero at the end of their turn.  Each turn they can either take the top card off the discard pile or draw a card off the top of the draw pile and then they choose and discard one card.  Their hand must equal zero after the discard for them to be the winner.

Add-up or Subtract Down:

When I am working with younger students, I don't want to complicate the rules too much with a game like 99, so we will just start at 0, and use card A-10.  Students play cards and only add them until we get to the target number (I usually start with 100, but we've played 250 before) and, again, the player who plays the card that pushes the pile over the target loses.  This works exactly in reverse for subtraction where you start at a number and subtract until someone forces the pile below 0, and they lose the game.  I also combine them, so we start at 0 and the student can choose to add or subtract whenever they play a card and I make the target 50 or 75 or maybe 100. 

Index cards with numbers written on them are a great proxy for playing cards if you are not allowed to use cards, or are just looking for some flexibility, because you can write whatever you want on index cards.