Valve has just announced its Counter-Strike: Global Offensive 'Operation Hydra' event, which will take place on a weekly basis until September. These events will feature "twists on the classic game rules", and will play out on new maps across both casual and competitive.
These events will mostly involve the aforementioned "twists". For example, Wingman, is a 2v2 best-of-16 format, while Weapons Expert is a 5v5 best-of-30 match where the player can only purchase a weapon once. Other varieties come in the form of War Games, which includes Heavy Assault Suit – a bomb defusal round with the added twist that one player is wearing heavy armor.
Meanwhile, Headshots Only is a War Game which does what it says on the tin; Hunters-Gatherers is a mode where each player drops a dogtag on death, which can be collected to win the game, and Stab Stab Zap involves only a knife, a recharging Zeus and grenades. There are more War Games, detailed over here.
Of course, you can buy an "all access" pass for $5.99 (US), which boosts the XP you earn, while also adding a new Guardian campaign where you and a friend play through a series of missions where, at a hunch, enemies will need to be shot. There are also the usual range of new weapon skins, cases and more. The full rundown can be read over here.