Some explanation for those who don't know:
One of my favorite game development companies, founded by the loveable charming geek Harvard dropout Gabe Newell, who has been subject to plenty of weight jokes and been a good sport. Valve is also known for delaying games (and not speaking about them for a really long time, aka Half Life 3). In addition to that, they have unique business methods such as horizontal structure (desks are on wheels and move constantly) and autonomy (teams are left to do what they want to without being bossed around excessively by those at the top).
It's a small company with less than 500 employees (though it was under 250 for a while), but they're working on a light-weight gaming console called the Steambox, to move PC's into the living room. The program Steam has dominated PC for a while, great sales on there.
But they've released solid games:
The Half Life Series
Portal, Portal 2
Team Fortress 2
The Counter Strike series
Left 4 Dead 1 and 2
I think in the next 5 years, considering their huge profits and successes in a short period of time, it's a possibility Valve will be up there with Sony and Nintendo, while Microsoft and it's Windows 8 plunge.