Free

AS Spider
Windows game that can use any jpg or bmp picture as the background for the deck.
http://www.phnet.fi/public/aki.suuronen/Games/Games.html

Cribbage Squares
Score as many points as possible by creating eight cribbage hands in a 4x4 grid. (Windows)
http://www.gamesbymax.com/cribsqrs.html

Dr. WuWu's Catch 24
Windows game where the aim is make twenty-four using four cards through addition, multiplication, subtraction, and division.
http://www.drwuwu.com/catch24.html

Free Spades
Free Spades, Hearts and Euchre games.
http://www.freespades.com/

FreeDoko
Windows or Linux game similar to Skat for four or more players.
http://free-doko.sourceforge.net/en/

Kvitlech 2000
Download an offline version of the game.
http://www.gvaldig.addr.com/kvitlech/

Mah-Jong
An open source version of the classical Chinese game for Unix or Windows, including network play and computer opponents.
http://www.stevens-bradfield.com/MahJong/

MathWar
A War variation for Linux in which you must multiply the values on the cards in able to be able to take the trick.
http://webpages.charter.net/stuffle/mathwar/MathWar.html

Mystic Island
Spades, Euchre, Hearts and Pinochle online games and multi-player tournament. Free download and play.
http://www.mysticisland.net/

Nate's Kentucky Rook
Game played with a trick deck against computer opponents. Includes a download and forums.
http://webpages.charter.net/nebsoftware/

Pidro Challenge
Provides a download, rules, FAQ, and users manual. [Windows]
http://www.pidrochallenge.com/

Pokermi
Download free poker game to play against your computer or others over the Internet.
http://uk.geocities.com/kamal_bahrin/

Scout's Game
An easy to learn game for Windows which can be played against the computer or against up to three other players over the internet.
http://rockxware.dse.nl/scout/

Shithead
Java version can be played single or multiplayer.
http://home.pacific.net.au/~cavenagh/SH/

Spi Poker
Play Holdem, Omaha, and Seven Card Stud against other players on the internet.
http://spi-poker.sourceforge.net/

Stackomania
Win by putting a stack of cards in order from lowest to highest. Play against a computer or a human opponent.
http://www.geocities.com/stackomania/

Thanos Cardgames
Over twenty downloadable card games.
http://www.geocities.com/thanoscardgames/