Cards against Humanity - A party game for horrible people.
Cockatrice - Cockatrice is an open-source, multiplatform program for playing tabletop card games over a network.
Web Trumps - A nerdy card game all about startups
Hearthstone - Pick up your cards and throw down the gauntlet!
Pitch Cards - I've 99 problems and I'm ready to pitch one!
XMage - XMage allows you to play Magic against one or more online players or computer opponents.