I had never heard of Microsoft Communicator (or Lync) until I saw it on my E7 and I don't know anybody who uses it.
Is it any good? I'm trying to work out how to set it up, but if its crap, i won't bother.
Communicator
Communicator is an IM tool for business use. You need a sever to connect to in order to use it so if your employer doesn't use it there's not much point in having it.
It is a Microsoft acquisition. Microsoft has had at least MSN Messenger, then they had NetMeeting, which was originally an acquisition, that they then replaced with a new version that became Live Meeting, or whatsitsname, that also replaced the original MSN Messenger), and apparently it wasn't good enough, so they acquired Lync. Now they've also acquired Skype. Let's which one of the three (Live, Lync, Skype) is still around a couple of year from now (unless they decide they need yet a new one for some reason or another).