What's to say, it's the Orange Box..!
-Half Life 2: Still my favorite shooter of all time. It's intelligent, beautifully designed, and downright fun (common qualities of steam games). The environments and guns are all very well thought out and intuitive, and it's probably the only game I've ever played where character interaction is realistic. They really took their time on this project, and the level of attention to detail shows and delivers.
-Team Fortress 2: This is my first venture into the TF universe, and I'm pleasantly surprised. Although not quite as complex or instantly gratifying as other shooters like COD4, TF2 has its own charm to it. The visual style is absolutely top-notch, and the class system is fun. There are also many interesting game modes to freshen things up.
-Portal: What's to be said that hasn't been already... It's Portal, I mean, it practically has a cult following. Easily one of the greatest games ever created.
In summary: buy the orange box.. now..