The Triple Entente: Britain, France, Russia
The Triple Alliance: Germany, Austria-Hungary, Italy. (However, Italy's membership was meaningless and Italy fought alongside the Entente from April 1915)
The Ottoman Empire, which fought alongside the Central Powers (from September 1914 on, was not a member of either.
The US, which entered the war on the side of the Triple Entente was also not a member of either.