Shoutbox

I've always been a fan of Outlook.  2010 is a particular improvement as it now uses the same 'ribbon' interface as the whole Office suite (something that was noticeably lacking in 2007).

It has a very nice 'conversation' view to see the history of e-mails.

The only fault for me is that I only use IMAP accounts and they don't support e-mail notification by default.  You have to manually add sounds/desktop alerts via 'rules'.....and it won't change the icon on the taskbar as it should.  The notification envelope is gone and instead it should change the icon of the program on the taskbar, but this doesn't happen in IMAP