It should be in Communications
Anywho, the obvious answer is the successor to MSN, Live Messenger
Although even counting just AIM and MSN 7.5, MSN is just full of so many more features. Better smilies, Winks, Nudges, Themes, Backgrounds, good interface, better sign on support (just need an e-mail and 5 seconds to get a Passport for it, rather than a Screen Name and the Advertisements that come with it for various useless AOL Services), Spaces integration, proper file transfer, proper Avatars, Private Messages as well as proper Nicknames, chromeless window, Contact Groupings, Organisation, more Options, Games, and not to mention, a way of actually seeing who is online rather than just reading through a list
Not even including the new stuff with Live Messenger