Left 4 Dead 2 (L4D2), the sequel to the best-selling and critically-acclaimed action thriller from Valve is being shown this weekend at The Penny Arcade Expo (PAX) in Seattle, Washington.

The game is available to play, by the press and public, at Valve and Microsoft's Xbox 360 booth.

After debuting "The Parish" campaign (set in the New Orleans' French Quarter) at E3 and "Swamp Fever" at Comic Con San Diego, Dark Carnival is the third of the five campaigns shipping with L4D2 this fall.

Valve is also unveiling a new Boss infected monster coming in the full version of Left 4 Dead 2. Known as "The Jockey" this is the third new Boss infected from L4D2 revealed thus far (the Charger was revealed at E3, The Spitter at Comic Con). The Jockey lets you attack lone Survivors and wrestle control of their movement away from them-into fire, off ledges, or straight into the middle of a zombie horde.

In addition to the new campaign and boss monster, the PAX showing of L4D2 marks the debut of the "Zombie Clown" (the "uncommon common" exclusive to the Dark Carnival campaign) and a few more of the 20-plus new items coming in the full product (electric guitar, machete, and the American baseball bat, of which the bat is being offered as a pre-sell exclusive with select retailers).

Coming exclusively to Xbox 360 and PC on November 17, Left 4 Dead 2 promises to set a new benchmark for co-operative action games and become one of 2009's marquee titles.