I think we need a thread for all card games EXCEPT poker!!! (It has enough attention at the moment).
Here are some of my favourites...
Bridge (when you have four)
500 (best in four)
Cribbage (2 player is best but works in 3)
Euchre (cut-throat 3 player is good)
Hearts (best in 4)
Spades (best ni 4 but 2 player works too)
Whist (particularly 2-player)
Canasta (4 is best from memory)
Rummy (ok in nearly any number up to 6)
Gin Rummy (2 only)
Uno (which is a rip-off of Crazy Eights and good in variable number but I think >3 is best)
Solitaires...
Klondike is the original and a favourite. I also played a games called Devil's Despair which had the advantage that you can usually play on a briefcase while sitting on a train.