Cockatrice - Cockatrice is an open-source, multiplatform program for playing tabletop card games over a network.
Evoker: Magic Card Game - Evoker: Magic Card Game is a Collectible Card, Action, Combat, Single and Multiplayer video game created and published by Flare Games.
XMage - XMage allows you to play Magic against one or more online players or computer opponents.
ForgeMtG - Forge is an implementation of Magic the Gathering that lets you play against a computer AI opponent using most of the rules of a real game. You can construct decks for you and the computer to play with, or play in a draft or a sealed deck format.
OCTGN - OCTGN (Online Card and Tabletop Gaming Network) is a virtual table top with an integrated player lobby and chat system that supports custom card and board games.
Magarena - Magarena is a single-player fantasy card game played against a computer opponent.