Just added MailTabs http://simplicity.fm/mailtabs/ to Apple Mail and it makes life much easier, adding tabs for opened and new emails instead of spawning new windows each time.
"MailTabs for Mail.app is a unique plug-in that organizes all your open email messages under a simple tab arrangement just like the tabs in your browser."
It's "donationware" so do consider the work put in by the developer and give a donation...
In conjunction with WideMail http://widemailplugin.com/ you get a very flexible, easy to use mail application. WideMail gives a three-pane window, showing the mailboxes, message list and a preview of the selected message.