Xbox, on a Phone
I'll admit, I very nearly needed to change my pants when I saw the Xbox tile on the phone for the first time. Obviously, you're not going to be playing Halo 3 on your smartphone (at least not this year), but yes, Xbox Live on a phone! It's tied to your Live profile, and there are achievements and gamer points for the games you can play on your phone, which will be tied to games back on your Xbox 360.
If Microsoft's got an ace-in-hole with Windows Phone 7, it's Xbox Live. Gamers have talked about a portable Xbox for years—this is the most logical way to do it. The N-Gage was ahead of its time. (Okay, and it sucked.) The DS and PSP are the past. The iPhone showed us that the future of mobile gaming was going to be on your phone, and now that just got a lot more interesting. The potential's there, and hopefully the games will be plentiful and awesome enough to meet it.