Valve Corporation, also known as Valve Software or Valve, is an American video game development and digital distribution company made famous for its multiple critically acclaimed video game series Half-Life, Portal, Left 4 Dead and Team Fortress, and its social-distribution networking software Steam.
The quote at the bottom of the advert, "Ordinarily, I wouldn't contemplate them... But these *are* extraordinary times," suggests that the collaboration will be based around, or at least launched via the Valve intellectual property Half-Life. Ashley Wood clarifies on 3A Forums: "refers [to] all valve, ALL. and shit its gonna be epic" 
The first item that has been revealed is a Weighted Companion Square² which, in a humorous sense, poses as a teaser of the collaboration between ThreeA and Valve.