Windows Live Mail, the email application. I did use Outlook Express for email, but that program messed up on me at one point, so I switched programs.
But, I'm still on MSN/Windows Live Messenger. For some reason, they haven't made me update yet, despite it being past the posted date. But, they're trying to trick me into switching over. Every time I wake up my computer after it falling to sleep, I get a pop-up asking if I want to upgrade it now. They'll probably eventually make it mandatory.
I never got email notifications from the messenger for my email addresses. For awhile, it did tell me I had one or two unread emails, and it was really annoying because I didn't know what it was talking about and couldn't find any way of finding out. Not long ago, my brother told me something about an online inbox they give people, and I went to look, and that's where those email things were. I never even knew about it, so it won't bother me if it's gone when switching to Skype.