Season 5 begins soon and season 6 is already slated as well. And there is no shortage of material for HBO to continue beyond that. George R.R. Martin currently has five novels published in the "Song of Ice and Fire" series, including A Game of Thrones (Season 1), A Clash of Kings (Season 2), A Storm of Swords (Season 3 and 4), A Feast for Crows and A Dance with Dragons. He's also working on two more books, the next of which is The Winds of Winter.
Game of Thrones Season 5: Jon & Mance (HBO)
Game of Thrones Season 5: Tyrion & Varys (HBO)
Official Website: http://www.hbo.com/game-of-thrones