While No More Heroes' Travis Touchdown isn't the most fitting crossover possibility for a Valve game, there's still a fair amount of joy to be had from seeing him be a credit to team.
Some modders have managed to insert Suda's leathery star into two of Valve's more recent titles, Left 4 Dead and Team Fortress 2.
You can see the Left 4 Dead one here. It's by DethKlan, and is just fine.
Finer, though, is the Travis Touchdown mod for Team Fortress 2 (by MechaWare), which not only sees him dropped into the game, but he gets an energy drink and custom baseball bat as well.
How Travis met Zoey: No More Heroes invades Valve-world [Destructoid]