Rules:
Backgammon
Chouette
Match play
Variants:
Acey-Deucey (Ace-Deuce)
Crazy Narde (Gul Bara)
Dutch Backgammon
Gul Bara (Crazy Narde)
Hypergammon
Longgammon
Nackgammon
Narde (Feuga)
Russian Backgammon
Snake
Tapa

Software:
JellyFish

Play on-line (no software download):
Against computer
  (After one game you can play off-line)
Human opponents 1
Human opponents 2
Human/robot opponents
By e-mail (including variants)

Software for on-line play:
Backgammon (Human opponents)