Yeah, Steam. Most vocal are the haters, but I just fired up a random mod linked in tnf's thread and... damn. Steam is the fucking bomb.
Think about it. Having a central launch pad for all your games, which updates them as necessary without your involvement.
Yeah, alright, everything beyond that is gravy, and some of it is quite irritating gravy. Having to decrypt game files is bullshit (although you can back them up), as is firing it up for the first time in months and finding you've got a couple of hundred megs to download before you can play (although the update process can be turned off individually per game, and halted mid-update if needed).
My biggest gripe is not having non-valve games on there, although that's quite obviously never going to change. Having an independant Steam clone would be awesome, but unlikely