We like to joke about Valve and their apparent inability to count to three. They've made sequels to Half-Life, Left 4 Dead, Portal and Team Fortress, but a third entry is yet to be made for any of these franchises.
But now it seems that Left 4 Dead 3 could have been teased, accidentally or deliberately (we're not sure), by a Valve employee. Conceptual artist Tristan Reidford posted a screenshot of his computer screen for a tutorial where a folder named "left4dead3" could be spotted. Since then the picture has been replaced but thanks to a fan on ValveTime, it has since been immortalised.
Sure, this could mean nothing at all. The folks at Valve may have folders of every possible future project they could feasibly take on in the future. But we like to hold on to hope, as we really liked Left 4 Dead and it's sequel.