Outlook has conversation view as well, and Microsoft Lync (formerly Office Communicator) integrates the rest of the feature set (yes you're vendor-locked, but you are with GMail too). In my opinion GMail executes better on most of these features, but they're not the only ones to offer them.
Not a fan, but Lync even allows XMPP access (if so configured), so all your GTalk buddies can message you from GMail (at least that should work - didn't try).
