There are lots of services that do lots of things including friends,achievements,offers,content downloads,and more.
But these are my favorite gaming services on pc...
1.Games For Windows Live
I like this service because it is like Xbox 360 but its on PC.This service includes achievements,gamerscore,friends,LIVE parties,cross-platform play,voice chats,spectating,add-on content,messaging,and other features that are also on Xbox Live.
This service is free so if anybody wants to give it a try,download it off the website.This service includes achievements,games on-demand,friends,clouds,chat rooms,and voice chat.Onlive even has a app on iPad and Android devices and their own game console!If you want the system,it requires Ethernet.
Everybody who plays on PC should be playing games from this service.It includes achievements,friends,chating,gifting,level editors,workshop,and captions.DOWNLOAD IT NOW if you are a PC gamer.
This service has little to offer because it has no achievements,messaging,or any of that stuff.But I like it because it is available on PC and Apple devices!
This service is cool because it has the awesome games that most people do not make.It also includes 89 percent of the things you can do on Steam.