Original Card Games by David Parlett
Players 2-5   Cards 52+jokers   Type Arithmetical
I really did invent this one in my sleep. Or, at least, at 4am when I woke up and couldn't get back again. Inventing card games is more constructive than counting sheep.
Cards
52 plus Jokers - two or four of them if two play, two if three play, four if four, or three if five. Deal all the cards around so that everyone gets the same number.
Object
To win cards, especially face cards (Jacks, Queens, Kings, Jokers), by combining cards that together yield a number ending in 1. That is, 1, 11, 21, etc, up to 10,001.
Play
Hold your cards face down in a pile. At each turn you all simultaneously take the top card of your pile and play it face up to the table - or, if two play, take the top two. You now try to use the value of any or all of the cards showing to make up a total ending in 1. Do this by adding, subtracting, multiplying or dividing, though division is allowed only it yields a whole number without remainder. For this purpose numerals Ace to 10 count 1 to 10 respectively, face cards count either 1 or 10 as you prefer, and a Joker counts exactly the same as any one of the other cards on display. In making up a calculation, you may not merely string numerals together. For example, you can't count a 3 and a 7 as either 37 or 73.
As soon as you can, you call out the highest such number you can make. If nobody can compile a higher number, or the same number but involving more of the cards in play, you win all the cards played, including any not involved in the calculation. Otherwise, they are won by the player who improved on it.
If you can't produce a one-up number, and believe no one else can, you call out "One off". If then nobody comes up with an answer, you who called turn the next card of your pile and add it to those already there. The same procedure is repeated until somebody does produce a number ending in 1 from the cards on display, except that anyone who called "One off" incorrectly is barred from contributing and so cannot win the cards.
If you make an incorrect call - whether an incorrect calculation or an incorrect call of "One off" - you are barred from calling again for the same display, and play continues until somebody makes a correct call.
Ending and scoring
Play continues until at least one person plays the last card from their hand and the cards on display have been won. You each score the number of cards you have won minus the number of cards still left unplayed in your hand (if any). For greater interest, score 1 per numeral, 2 for a Jack, 3 for a Queen, 4 for a King, and 5 for a Joker. These enhanced values apply only to cards won, not to cards left in hand. Play any previously agreed number of deals or up to any agreed target score.
Highest possibles
The highest possible number in a five-player game is 10,001, made when the displayed cards are five face cards, or four faces and an Ace (10x10x10x10+1). Similarly, the highest when two or four play is 1001, and with three it's 101.
Footnote
If you like equation games, see also Brain Drain, Equator, and Prime Time.