Valve seems intent on making PC gaming a better fit for the living room. Steam's Big Picture mode is part of that effort, and the super-sized UI works very well from across the room... until you need to install one of those DirectX runtimes before playing a new game. The installer tends to pop up an old-school Steam window, interrupting the Big Picture experience. Games that offer official Big Picture support appear to address that issue, and Portal 2 is now among them. The latest Portal 2 update adds Big Picture support in addition to a complementary treat: split-screen multiplayer.
Screen-sharing multiplayer modes are common in console games but rarely seen on the PC side. Of the games I've played recently, only Serious Sam: BFE has accommodated two players on the same screen. There appears to be a way to hack split-screen functionality for Borderlands 2, but limitations are attached and additional software is required.
Those three games are very different, but they all feature cooperative multiplayer modes that are especially well-suited to screen sharing. Adversarial deathmatches are less ideal, since you can easily glance at your opponent's portion of the screen. Split-screen combative play can still be a lot of fun, though; just look at the four-way GoldenEye 007 multiplayer on the Nintendo 64.
Split-screen gaming isn't perfect, of course. Your window on the world becomes smaller, and it usually feels too narrow or too wide depending on whether the screen is split vertically or horizontally. Still, allowing multiplayer players to share the same screen—and the same system—is an important feature if PCs are to challenge consoles in the living room. I'd rather multiple players bring their own PCs for a proper LAN party, but that's not always convenient. Thanks to Joystiq for the tip.