Cockatrice - Cockatrice is an open-source, multiplatform program for playing tabletop card games over a network.
Cards against Humanity - A party game for horrible people.
Hearthstone - Pick up your cards and throw down the gauntlet!
Web Trumps - A nerdy card game all about startups
XMage - XMage allows you to play Magic against one or more online players or computer opponents.
Mystic Mondays - The magic of tarot in your pocket