World of Warcraft or StarCraft II; Blizzard is currently offering both titles and their expansions at a deep discount. New players to WoW can grab the base game, with all expansions up until Mists of Pandaria, for only $4.99. To add MoP to the mix, plunk down another $9.99 for the standard edition or $29.99 for the digital deluxe edition (a savings of $30.00 on each). StarCraft II fans can save twenty bucks and scoop up Wings of Liberty for $19.99 and/or Heart of the Swarm for $19.99 or $39.99 (standard and deluxe editions, respectively).
For those who already have the games but want to get friends involved, there is also an option to gift the purchase. However, to take advantage of the deals you might want to act fast -- there's no guarantee that the sale will last.