Fans of the popular and highly addictive Left 4 Dead (I include myself in that group), will be excited to hear that Valve has just announced Left 4 Dead 2. In addition to new characters, zombies, and special infected, L4D2 will introduce melee combat weapons like a chainsaw, frying pan, or baseball bat (among others), and a new AI director that will dynamically adjust level layout, weather, lighting, and other objects in the game world.We’ve got a teaser trailed posted below, along with a summary of the game’s